Drilled porous resin base material, and method of manufacturing porous resin base material with conductive drilled inner wall surface

ABSTRACT

A production process of a perforated porous resin base, comprising Step 1 of impregnating the porous structure of a porous resin base with a liquid or solution; Step 2 of forming a solid substance from the liquid or solution impregnated; Step 3 of forming a plurality of perforations extending through from the first surface of the porous resin base having the solid substance within the porous structure to the second surface in the porous resin base; and Step 4 of melting or dissolving the solid substance to remove it from the interior of the porous structure, and a production process of a porous resin base with the inner wall surfaces of the perforations made conductive, comprising the step of selectively applying a catalyst only to the inner wall surfaces of the perforations to apply a conductive metal to the inner wall surfaces.

TECHNICAL FIELD

The present invention relates to a production process of a porous resinbase (hereinafter may also be referred to as “porous synthetic resinbase”) perforated through the thickness of a porous resin material. Thepresent invention also relates to a production process of a porous resinbase with the inner wall surfaces of the perforations selectively madeconductive.

The perforated porous resin base obtained by the production processaccording to the present invention can be utilized in a wide variety offields of, for example, materials for connection between circuits,anisotropically conductive materials and insulating materials in anelectronic field; medical devices such as patch repairing materials in amedical field; and separation membranes.

The porous resin base with the inner wall surfaces of the perforationsmade conductive, which is obtained by the production process accordingto the present invention, can be used in, for example, electricalconnection between circuit devices in semiconductor devices; and testsfor electrical reliability, which are carried out in circuit boards,semiconductor wafers and semiconductor packages.

The porous resin bases have various forms including a sheet, a tube anda block and are preferably in the form of a sheet. In the presentinvention, a porous resin sheet means not only a sheet having athickness of 0.25 mm or greater, but also a film having a thicknesssmaller than 0.25 mm.

BACKGROUND ART

In a field of electronics, for example, phenol resins, epoxy resins,glass epoxy resins, polyimide resins, polyester resins, polysulfoneresins and polytetrafluoroethylene resins have heretofore been used asresin substrates from the viewpoints of mechanical strength, electricalproperties, heat resistance and the like (for example, “ELECTRONICSJISSO GIJUTSU KOZA (Elementary Course of Mounting Techniques inElectronics), Vol. 1, Introduction”, edited by Association of HybridMicroelectronics, Kogyo Chosakai Shuppan, issued on Jul. 1, 1998,Chapter 4, pp. 203-209).

In recent years, with the use of still higher frequency and speeding-upin the field of electronics, particularly, a lower dielectric constanthas been required as material properties required of substrates.Attention has been attracted to porous resins as materials for resinsubstrates because they are low in dielectric constant compared withordinary non-porous resin materials.

A material for connection between circuits or an anisotropicallyconductive material has a structure that perforations (hereinafter mayalso be referred to as “through-holes”) are provided in necessaryportions of a substrate, and inner wall surfaces of the perforations arecovered with a conductive material. In order to use a porous resinmaterial as a material for these substrates, it is thus necessary toform perforations greater than the pore diameter of the porous resinmaterial.

In general, methods for providing perforations (through-holes) in asubstrate include machine-working methods, for example, punching by apunch and a die, blanking by a die, and perforating by a drill (forexample, “MAIKURO KAKO GIJUTSU (Microworking Techniques)”, edited by theEditorial Committee of Microworking Techniques, The Nikkan KogyoShinbun, LTD., issued on Sep. 30, 1988, Chapter 1, pp. 8-13, Chapter 2,pp. 168-175). A method of perforating by ultrasonically vibrating thetip of a tool, a chemical etching method, in which a chemical corrodingaction is utilized to perforate, and a light-abrasion method, in whichlaser beams are irradiated to perforate, are also known as perforatingmethods.

When a porous resin material (hereinafter referred to as “porous resinbase”) formed in the shape of a substrate is perforated by amachine-working method, however, the base itself is deformed, the porousstructure of edges and inner wall surfaces of perforations is collapsed,and burr occurs on opening portions of the perforations, so that it isextremely difficult to form perforations with high precision whileretaining the porous structure. Even when the method of perforating byultrasonically vibrating the tip of a tool is applied to the porousresin base, it is difficult to form perforations with high precision.

When the porous resin base is perforated by irradiation of laser beams,peripheries of perforated portions are melted and deformed by heat, orthe porous structure of edges and inner wall surfaces of perforations iscollapsed. The chemical etching method permits a porous resin base to beperforated according to the kind of the resin forming the porous resinbase. However, this method is unsuitable for a method for perforating aporous resin base composed of a corrosion-resistant resin. The porousresin base has a possibility that it may be perforated by irradiation ofshort-wavelength laser beams such as excimer laser. However, it takes along time to work it, and so the cost thereof is expensive.

When the porous structure of edges and inner wall surfaces of theperforations in the porous resin base is collapsed, the propertiescharacteristic of the porous resin material are impaired. The porousresin base has elasticity in a thickness-wise direction thereof. Whenthe porous structure about the perforations is collapsed, however, theperforated portions are collapsed by only applying a compressive load tothe porous resin base once to lose the elasticity.

When the porous resin base perforated is used as a material forconnection between circuits or an anisotropically conductive material,it is necessary to make the inner wall surfaces of the perforations in athickness-wise direction conductive by applying a conductive metal suchas plating particles to them. When the porous structure about theperforated portions is collapsed, however, it is difficult to apply aplating catalyst. In addition, when the porous structure about theperforated portions is collapsed, the elasticity of the conductiveportions is impaired even when the inner wall surfaces of theperforations are made conductive, so that the conductive portionsthemselves are collapsed when a compressive load is applied.

Further, even when the porous resin base is perforated, it is extremelydifficult to selectively apply a conductive metal only to the inner wallsurfaces of the perforations by a subsequent secondary working to makethem conductive. As described above, it is difficult to preciselyperforate the porous resin base, and the secondary working subsequent tothe perforating is also difficult. These problems are specificallydescribed taking an anisotropically conductive sheet (hereinafter mayalso be referred to as “anisotropically conductive film”) as an example.

In the field of electronics such as semiconductor devices, ananisotropically conductive sheet capable of imparting conductivity onlyto a thickness-wise direction thereof is used as a means for compactlyconducting electrical connection between circuit devices. For example,the anisotropically conductive sheet is widely used for compactlyconducting the electrical connection between circuit devices withoutusing a means such as soldering.

There has also been proposed a method of interposing an anisotropicallyconductive sheet between electrodes to be inspected and electrodes of aninspection apparatus for the purpose of achieving electrical connectionbetween the electrodes to be inspected formed on one surface of acircuit board, which is an object of inspection, and the electrodes ofthe inspection apparatus. This anisotropically conductive sheetpreferably has elasticity in the thickness-wise direction thereof forthe purpose of achieving the electrical connection between theelectrodes to be inspected and the electrodes of the inspectionapparatus without damaging the electrodes to be inspected and byabsorbing variations of height among the electrodes to be inspected.

As specific examples of the anisotropically conductive sheet, there hasbeen proposed, for example, an anisotropically conductive material forconnection obtained by dispersing conductive particles in a bindercomposed of an epoxy resin to form a sheet (for example, Japanese PatentApplication Laid-Open No. 4-242010). This anisotropically conductivematerial for connection is so constructed that when the conductivematerial is pressed between terminals opposed to each other, theconductive particles come into contact with the respective terminalsonly at compressed portions to conduct only in a thickness-wisedirection between the terminals. The dispersed state of the conductiveparticles is controlled, thereby retaining the insulating property in alateral direction of the sheet.

There have also been known anisotropically conductive sheets obtained byforming a great number of through-holes in a sheet formed from apolymeric material and filling a conductive material into the respectivethrough-holes to make only specified portions of the sheet in athickness-wise direction thereof conductive. There have been proposed,for example, anisotropically conductive sheets obtained by filling aninsulating elastic polymeric substance, in which conductive particleshave been dispersed, into each of a plurality of through-holes providedin an insulating plate formed from a resin material or a composite resinmaterial reinforced with glass fiber and having stiffness to provideconductive path-forming devices (for example, Japanese PatentApplication Laid-Open No. 9-320667).

There have been proposed electrically connecting members obtained byforming a great number of through-holes in an electrically insulatingpolymeric film and filling a metal into the respective through-holes tomake the film conductive only in a thickness-wise direction of the film(for example, Japanese Patent Application Laid-Open No. 2-49385), andelastic connectors obtained by arranging a conductive member within aplurality of through-holes formed in a thickness-wise direction of anelastic sheet member subjected to a foaming treatment (for example,Japanese Patent Application Laid-Open No. 2003-22849).

In the anisotropically conductive sheets having the structure that theconductive material is filled into the respective through-holes in thesheet formed from the polymeric material, as a method for forming thethrough-holes (perforations), is adopted, for example, an etching methodmaking use of a light source such as a laser, or a machine-workingmethod such as pressing, punching or drilling. According to the etchingmethod, small through-holes having a hole diameter of at most 100 μm,further at most 50 μm can be generally formed. However, this method isexpensive in working cost. The machine-working method is generally usedin the case where relatively large through-holes having a hole diameterof at least 100 μm are formed and has a feature that it is cheap inworking cost.

On the other hand, the anisotropically conductive sheet desirably hassufficient elasticity to achieve connection between electrodes to beconnected or electrodes to be inspected without damaging them and absorbvariations of height among electrodes to be inspected to achieve goodelectrical connection. An anisotropically conductive sheet havingelasticity in a thickness-wise direction thereof and permittingconduction in the thickness-wise direction under a low compressive loadcan be used repeatedly in inspection of electrical conduction because ithas elastic recovery property in addition to the fact that it scarcelydamages the electrodes to be inspected.

The anisotropically conductive sheets, in which an elastomer with theconductive particles dispersed therein or the metal filled into therespective through-holes in the sheet formed from the electricallyinsulating polymeric material to provide conductive portions (conductivepaths), involve such problems that a high compressive load is requiredfor achieving conduction in the thickness-wise direction and that theelasticity at the conductive portions is deteriorated due todeterioration of the elastomer with time or upon use under a hightemperature in a burn-in test or the like.

In the state of the art, it has however been difficult even by thoseskilled in the art to use a porous resin base having elasticity in athickness-wise direction thereof to form perforations with highprecision without collapsing the porous structure and further to subjectthe porous resin base to a secondary working such as selectiveapplication of a conductive metal to inner wall surfaces of theperforations.

On the other hand, in a medical field, an expanded porouspolytetrafluoroethylene (hereinafter abbreviated as “expanded PTFE”) isused in artificial blood vessels and medical devices such as patchrepairing materials and sutures. The expanded PTFE has highly inertchemical properties and moreover has such properties that the internalgrowth of vital tissues is allowed by controlling a microstructure thatthe porous structure is formed. The expanded PTFE is known to facilitatethe internal growth of vital tissues by providing microscopicperforations extending through in a thickness-wise direction thereof.

There have heretofore been proposed expanded PTFE sheet materials havinga microstructure comprising nodes connected to each other by fibrils andhaving microscopic pores extending through in a thickness-wise directionthereof (for example, Japanese Patent Application Laid-Open (KOHYO) No.8-506777 (through PCT route)). In this document, it is described thatwhen an expanded PTFE material subjected to expanding before perforatingis perforated by a needle, the perforations have very rough edgesappearing to be caused by irregular cutting and deformation of thematerial. This document also shows that perforating by removing theexpanded PTFE using a sharp blade also results in perforations havingrough edges. When the expanded PTFE material subjected to theperforating is used as a medical device such as a patch, there is apossibility that some trouble may occur in a vital body when theperforations have rough edges.

Thus, the document (Japanese Patent Application Laid-Open (KOHYO) No.8-506777 (through PCT route)) has proposed a method that the expandedPTFE material is not perforated, but an extruded product beforeexpanding is perforated and then expanded. More specifically, thisdocument discloses a process for producing an expanded PTFE materialhaving a microstructure comprising nodes connected to each other byfibrils and microscopically perforated, which comprises extruding abillet preliminary formed from a mixture of PTFE and a liquid lubricantto produce an extruded product, removing the liquid lubricant from theextruded product, forming microscopic pores extending through in athickness-wise direction of the extruded product and then uniaxially orbiaxially expanding the extruded product. This document describes thatwhen the extruded product before expanding is perforated and thenexpanded, an expanded PTFE material having perforations withsubstantially smooth edges is obtained.

According to the process described in the document (Japanese PatentApplication Laid-Open (KOHYO) No. 8-506777 (through PCT route)), theextruded product before expanding is perforated and then expanded,thereby smoothening rough edges caused by the perforating. However, thisprocess is insufficient to form perforations having edges highlysmoothened. In addition, according to the process described in thisdocument, the extruded product is perforated and then uniaxially orbiaxially expanded to form a porous structure. It is thus difficult tocontrol the positions and diameter of the perforations with highprecision.

A perforated porous resin base used as a substrate of a material forconnection between circuits or an anisotropically conductive material isrequired to preset the positions and diameter of a plurality ofperforations with high precision. Unless the positions of theperforations can be controlled with high precision, electricalconnection between circuit devices or electrical connection betweenelectrodes to be inspected of a circuit board and electrodes of aninspection apparatus cannot be precisely carried out by means of such aporous resin base even when a conductive metal is applied to the innerwall surfaces of the perforations to make them conductive.

Further, according to the process described in the above-describeddocument, an expanded PTFE material having perforations can be produced,but the process cannot be applied to selective application of theconductive metal to the inner wall surfaces of the perforations to makethem conductive.

DISCLOSURE OF THE INVENTION

A porous resin material having electrically insulating property, a lowdielectric constant and elasticity is suitable for use as a resin baseof materials for connection between circuits, anisotropically conductivematerials or the like. In order to use a porous resin base to produce amaterial for connection between circuits or an anisotropicallyconductive material, it is necessary to form through-holes(perforations) having sharp edges at necessary positions of the basewithout collapsing the porous structure thereof and causing deformationor producing burr.

Regarding this, description is given taking an anisotropicallyconductive sheet as an example. As described above, the anisotropicallyconductive sheet desirably has sufficient elasticity for the purpose ofachieving good electrical connection without damaging electrodes to beconnected and electrodes to be inspected and by absorbing variations ofheight among the electrodes to be inspected. The present inventors thusdeveloped an anisotropically conductive sheet of the structure that anelectrically insulating porous resin sheet is used as a base film,through-holes are provided at a plurality of positions of the base film,and a conductive metal is applied to the wall surfaces of thethrough-holes and previously proposed (see Japanese Patent ApplicationNo. 2003-096173).

An electrically insulating, elastic and porous resin sheet is suitablefor use as the base film of the anisotropically conductive sheet. When aprocess of applying a conductive metal to the inner wall surfaces of aplurality of through-holes provided in a porous resin sheet to formconductive portions is adopted in place of the process of filling aconductive material such as an elastomer, in which conductive particleshave been dispersed, or a metal into respective through-holes in a sheetformed from a polymeric material, there can be provided ananisotropically conductive sheet which permits conduction in athickness-wise direction thereof under a low compressive load inaddition to excellent elasticity in the thickness-wise direction, andmoreover can be used repeatedly in inspection of electrical conductionbecause the conductive portions can be restored to their original formby elastic recovery.

As a method for applying the conductive metal to the wall surfaces ofthe respective through-holes in the porous resin sheet, an electrolessplating method is suitable. In order to deposit conductive metalparticles only on the inner wall surfaces of the through-holes by theelectroless plating method to make them conduct, however, it isnecessary in a step of applying a catalyst (plating catalyst)facilitating a chemically reducing reaction prior to this plating tomask other portions than the inner wall surfaces of the respectivethrough-holes provided in the porous resin sheet to apply the catalystonly to the inner wall surfaces.

When, for example, a method, in which the same porous resin sheets asthe base film are laminated as mask layers on both sides of the basefilm, through-holes are formed in the resulting laminate, a catalystfacilitating a chemically reducing reaction is applied to the wholesurface of the laminate, including the through-holes, the mask layersare then separated, is adopted as a masking method, the catalyst appliedto the other portions than the inner wall surfaces of the through-holescan be removed together with the mask layers. When electroless platingis carried out using the catalyst applied to and remaining on the wallsurfaces of the respective through-holes after the removal of the masklayers, the conductive metal can be applied only to the inner wallsurfaces of the through-holes to form conductive portions (theabove-described Japanese Patent Application No. 2003-096173).

When a machine-working method, which is cheap in process cost, isapplied to the formation of through-holes as great as at least 100 μm indiameter in a porous resin sheet, however, the porous structure aboutthe through-holes including the inner wall surfaces thereof iscollapsed, so that it is difficult to sufficiently apply the conductivemetal to the inner wall surfaces of the through-holes by the electrolessplating. When the porous structure is collapsed by the perforating, theelasticity of the perforated portions is impaired.

It is an object of the present invention to provide a production processof a perforated porous resin base, by which perforations (through-holes)having smooth edges can be formed at necessary positions of a porousresin base with high precision without incurring collapse of the porousstructure, deformation of the base and occurrence of burr.

Another object of the present invention is to provide a productionprocess of a porous resin base made conductive by forming a plurality ofperforations (though-holes) in a porous resin base and applying aconductive metal to the inner wall surfaces of the respectiveperforations to form conductive portions, by which when the perforationsare formed by a machine-working method, perforations having smooth edgescan be formed with high precision while preventing collapse of theporous structure about the perforated portions, and moreover a platingcatalyst facilitating a reducing reaction of a metal ion can beselectively applied to the inner wall surfaces of the respectiveperforations, thereby surely applying the conductive metal to the innerwall surfaces by electroless plating or the like.

The present inventors have carries out an extensive investigation with aview toward achieving the above objects. As a result, the inventors haveconceived of a process comprising impregnating the porous structure of aporous resin base with a liquid or solution, forming a solid substancefrom the liquid or solution impregnated and forming a plurality ofperforations extending through from the first surface of the porousresin base having the solid substance within the porous structure to thesecond surface in the porous resin base.

According to the production process of the present invention, theperforations can be formed at the necessary positions without collapsingthe porous structure even when a mechanical perforating method isadopted. The perforations formed have smooth edges and do not causedefects such as deformation. After the perforating, the solid substancecan be removed from the interior of the porous structure by melting ordissolving it. As the liquid or solution impregnated into the porousresin base, may be used any of various substances such as water,alcohols, hydrocarbons and polymers.

It has also been conceived that when the above-described process isapplied, a porous resin base with a conductive metal selectively appliedto the inner wall surfaces of the perforations can be produced.

It has been found that the porous structure including both surfaces ofthe porous resin base is impregnated with a soluble polymer or paraffinto form a composite sheet, and solid layers of the soluble polymer orparaffin existing on both surfaces of the porous resin base are used asmasking materials, whereby after formation of a plurality ofperforations, a catalyst facilitating a reducing reaction of a metal ioncan be selectively applied to the inner wall surfaces of the respectiveperforations.

The soluble polymer or paraffin is impregnated as a liquid (melt) orsolution into the interior of the porous structure including bothsurfaces of the porous resin base. When this process is adopted, theporous structure about the perforated portions including the inner wallsurfaces is prevented from being collapsed even when the perforationsare formed by a machine-working method. The perforating is carried outat a temperature that the soluble polymer or paraffin impregnated intothe porous structure retains its solid state. When a substance that issolid at ordinary temperature (15 to 30° C.) is used as the solublepolymer or paraffin, the perforation can be formed at ordinarytemperature. The soluble polymer or paraffin soluble in a solvent can beeasily removed by dissolving it in the solvent after using it as themasking material. This process can also be carried out by using acompound capable of forming a solid substance by a chemical reaction,such as a polymerizable monomer, in place of the soluble polymer orparaffin.

The present inventors have further conceived of a process of using aliquid or solution containing a compound capable of forming a solidsubstance by a chemical reaction as another process for producing theporous resin base with the conductive metal selectively applied to theinner wall surfaces of the perforations.

More specifically, porous resin layers are laminated as mask layers onboth surfaces of a porous resin base to form a laminate of a 3-layerstructure, the respective porous structures of the laminate areimpregnated with a liquid or solution containing a compound capable offorming a solid substance by a chemical reaction, and the compound inthe liquid or solution impregnated is subjected to a chemical reactionto form a solid substance.

After a plurality of perforations extending through from the firstsurface of the laminate having the solid substance within the respectiveporous structures to the second surface are formed in the laminate, thesolid substance is removed. A catalyst facilitating a reducing reactionof a metal ion is applied to the surfaces of the laminate including theinner wall surfaces of the respective perforations. The mask layers arethen separated from both surfaces of the porous resin base, and thecatalyst applied to and remaining on the inner wall surfaces of therespective perforations in the porous resin base is used to apply theconductive metal to the inner wall surfaces, whereby a porous resin basewith the inner wall surfaces of the perforations selectively madeconductive can be produced. This process can also be carried out byusing the soluble polymer or paraffin in place of the compound capableof forming a solid substance by a chemical reaction. The presentinvention has been led to completion on the basis of these findings.

According to the present invention, there is thus provided a process forproducing a perforated porous resin base, which comprises the followingSteps 1 to 4:

(1) Step 1 of impregnating the porous structure of a porous resin basewith a liquid or solution;

(2) Step 2 of forming a solid substance from the liquid or solutionimpregnated;

(3) Step 3 of forming a plurality of perforations extending through fromthe first surface of the porous resin base having the solid substancewithin the porous structure to the second surface in the porous resinbase; and

(4) Step 4 of melting or dissolving the solid substance to remove itfrom the interior of the porous structure.

According to the present invention, there is also provided a process forproducing a porous resin base with the inner wall surfaces ofperforations made conductive, which comprises the following Steps I toIV:

(1) Step I of impregnating the porous structure of a porous resin baseincluding both surfaces thereof with a soluble polymer or paraffin, or acompound capable of forming a solid substance by a chemical reaction;

(2) Step II of forming a solid substance from the soluble polymer orparaffin, or the compound capable of forming a solid substance by achemical reaction, which has been impregnated, to form a composite sheetof a structure that both surfaces of the porous resin base have a layerof the solid substance, and the solid substance is impregnated into theporous structure;

(3) Step III of forming a plurality of perforations extending throughfrom the first surface of the composite sheet to the second surface inthe composite sheet;

(4) Step IV of applying a catalyst facilitating a reducing reaction of ametal ion to the surfaces of the composite sheet including the innerwall surfaces of the respective perforations;

(5) Step V of removing the solid substance from the composite sheet; and

(6) Step VI of using the catalyst applied to and remaining on the innerwall surfaces of the respective perforations in the porous resin base toapply a conductive metal to the inner wall surfaces.

According to the present invention, there is further provided a processfor producing a porous resin base with the inner wall surfaces ofperforations made conductive, which comprises the following Steps i toviii:

(1) Step i of laminating, as mask layers, porous resin layers (B) and(C) on both surfaces of a porous resin base (A) to form a laminate of a3-layer structure;

(2) Step ii of impregnating the respective porous structures of thelaminate with a soluble polymer or paraffin, or a compound capable offorming a solid substance by a chemical reaction;

(3) Step iii of forming a solid substance from the soluble polymer orparaffin, or the compound capable of forming a solid substance by achemical reaction, which has been impregnated;

(4) Step iv of forming a plurality of perforations extending throughfrom the first surface of the laminate having the solid substance withinthe respective porous structures to the second surface in the laminate;

(5) Step v of dissolving the solid substance to remove it from theinteriors of the respective porous structures;

(6) Step vi of applying a catalyst facilitating a reducing reaction of ametal ion to the surfaces of the laminate including the inner wallsurfaces of the respective perforations;

(7) Step vii of removing the mask layers from both surfaces of theporous resin base (A); and

(8) Step viii of using the catalyst applied to and remaining on theinner wall surfaces of the respective perforations in the porous resinbase (A) to apply a conductive metal to the inner wall surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged microphotograph of a perforation formed by aprocess according to the present invention for producing a perforatedporous resin base.

FIG. 2 is an enlarged microphotograph of a perforation formed by aprocess for perforating a porous resin base by means of amachine-working method.

FIG. 3 is a flow diagram illustrating the steps of a process accordingto the present invention for producing a porous resin base with theinner wall surfaces of perforations made conductive.

FIG. 4 is a flow diagram illustrating the steps of another processaccording to the present invention for producing a porous resin basewith the inner wall surfaces of perforations made conductive.

FIG. 5 is a schematic cross-sectional view of a testing equipment foranisotropically conductive sheets.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Porous Resin Base (Base Film)

As a resin material for forming a porous resin base used in the presentinvention, any resin may be used so far as it can form a porous resin.As the porous resin base, that formed from a resin material excellent inheat resistance, workability, mechanical properties, dielectriccharacteristics and the like is preferably selected for the purpose ofwithstanding a perforating method adopted in the present invention andapplication of a conductive metal and suiting uses in a field ofelectronics and a medical field.

In, for example, an anisotropically conductive sheet used in electricalconnection between circuit devices or inspection of electricalconduction, a base (base film) thereof is preferably excellent in heatresistance. In a burn-in test in particular, it is necessary to use thebase excellent in heat resistance because accelerated deterioration at ahigh temperature is conducted in a state that the anisotropicallyconductive sheet has been interposed between electrodes to be inspectedof a circuit device and electrodes of an testing equipment.

The anisotropically conductive sheet is also required to permit it to bemade conductive in a thickness-wise direction thereof and haveelectrically insulating property in a lateral direction (directionperpendicular to the thickness-wise direction) thereof. Accordingly, asynthetic resin forming the porous resin base, which will become a basefilm, is required to have electrically insulating property. Ananisotropically conductive sheet for electrical connection in particularis preferably that making use of a porous resin base formed of asynthetic resin having a low dielectric constant so as not to form thecause that signal delay occurs when a semiconductor device or the likeis used by high-frequency signals.

Examples of the synthetic resin forming the porous resin base includefluorocarbon resins such as polytetrafluoroethylene (PTFE),tetrafluoroethylene/hexafluoropropylene copolymers (FEP),tetrafluoroethylene/perfluoroalkylene copolymers (PFA), polyvinylidenefluoride (PVDF), polyvinylidene fluoride copolymers andethylene/tetrafluoroethylene copolymers (ETFE resins); and engineeringplastics such as polyimide (PI), polyamide-imide (PAI), polyamide (PA),modified poly(phenylene ether) (mPPE), poly(phenylene sulfide) (PPS),poly(ether ether ketone) (PEEK), polysulfone (PSU), poly(ether sulfone)(PES) and liquid crystal polymers (LCP).

Among these synthetic resins, fluorocarbon resins are preferred from theviewpoints of heat resistance, chemical resistance, workability,mechanical properties, dielectric characteristics (low dielectricconstant) and the like, with PTFE being particularly preferred.

In the production processes according to the present invention, forexample, a polymer or paraffin soluble in a solvent is used as a maskingmaterial, and a method of dissolving the soluble polymer or paraffin inthe solvent to remove it is generally adopted after a catalystfacilitating a reducing reaction of a metal ion is applied, so that thesynthetic resin forming the base is preferably insoluble or hardlysoluble in used in dissolving the soluble polymer or paraffin. This alsocorresponds to the case where a solid substance within the porousstructure is dissolved in a solvent to remove it in the productionprocess of the perforated porous resin base. A fluorocarbon resin ispreferred from the viewpoint of such behavior to the solvents, with PTFEbeing particularly preferred.

Examples of a method for producing the porous resin base includeperforating, phase separation, solvent extraction, expanding and laserirradiation methods. The form of the porous resin base may be suitablypreset from a sheet, a tube, a block and the like as necessary for theend application intended. In many cases, however, it is a sheet(including a film). For example, a porous resin sheet is used as a basefilm, whereby elasticity can be imparted to the resultinganisotropically conductive sheet in a thickness-wise direction thereof,and the dielectric constant thereof can be further reduced.

The porous resin base preferably has a porosity within a range of 20 to80%. The porous resin base preferably has an average pore diameter of atmost 10 μm or a bubble point of at least 2 kPa. From the viewpoint offorming conductive portions at a fine pitch, it is more preferable thatthe average pore diameter be at most 1 μm, or the bubble point be atleast 10 kPa.

The thickness of the porous resin base may be suitably selectedaccording to the purpose of use and a position used. However, it isgenerally at most 3 mm, preferably at most 1 mm, and the lower limitthereof is generally 5 μm, preferably 10 μm. The thickness of the poroussynthetic resin sheet in the case of, for example, an anisotropicallyconductive sheet for test of electrical conduction is of the order ofpreferably 5 to 500 μm, more preferably 10 to 200 μm, particularlypreferably 15 to 100 μm.

Among porous resin bases, an expanded PTFE sheet produced by anexpanding method is an excellent material as a base film foranisotropically conductive sheets because it is excellent in heatresistance, workability, mechanical properties, dielectriccharacteristics and the like and has an even pore diameter distribution.The expanded PTFE sheet is also suitable for use in medical devices suchas patch repairing materials.

The expanded PTFE sheet used in the present invention can be produced inaccordance with, for example, the process described in Japanese PatentPublication No. 42-13560. A liquid lubricant is first mixed withunsintered powder of PTFE, and the resultant mixture is extruded into atube or plate by ram extrusion. When a sheet having a small thickness isdesired, the plate-like extruded product is rolled by pressure rolls.After the extrusion and rolling, the liquid lubricant is removed fromthe extruded product or rolled product as needed.

When the thus-obtained plate-like extruded product or rolled product isuniaxially or biaxially expanded, an unsintered porous PTFE sheet isobtained. When the unsintered porous PTFE sheet is heated to atemperature of at least 327° C. that is a melting point of PTFE whilefixing it so as not to cause shrinkage, thereby fixing the ecpandedstructure by sintering, an expanded PTFE having high strength isobtained. When a tube-like extruded product is uniaxially expanded andsintered, an expanded PTFE tube is obtained. The expanded PTFE tube canbe formed into a sheet by cutting it in a longitudinal directionthereof.

The expanded PTFE sheet has a microstructure (hereinafter also referredto as “micro-fibrous tissue”) comprising a great number of very finefibrils and a great number of nodes connected to each other by thefibrils. In the expanded PTFE sheet, this microstructure forms a porousstructure. Accordingly, in the expanded PTFE sheet, a resin portion ofthe porous structure is composed of the fibrils and nodes, while theinterior (hereinafter also referred to as “void portion” or “poreportion”) of the porous structure is composed of spaces formed by thefibrils and nodes. The expanded PTFE sheet is excellent in elasticity ina thickness-wise direction thereof and also excellent in elasticrecovery property.

2. Impregnating Liquid or Solution (Impregnating Substance)

In the present invention, the porous structure of the porous resin baseis impregnated with a liquid or solution prior to perforating. Thisliquid or solution is that capable of forming a solid substance. Thissolid substance can be melted or dissolved. In the present invention, animpregnating substance is called a liquid or solution. However, it isrepresented by a state upon the impregnation. Accordingly, suchsubstances also include those in a solid state at ordinary temperature.

The liquid or solution is only required to be in the form of a liquid orsolution at the time it is impregnated into the porous structure of theporous resin base. For example, a substance having a high solidifyingpoint or melting point and being in a solid state at ordinarytemperature (15 to 30° C.) is only required to be impregnated into theporous structure of the porous resin base after it is heated into aliquid (melt). After the impregnation, it is cooled to a temperature ofits solidifying point or melting point or lower to solidify it.

A substance in a solid state at ordinary temperature is cooled to atemperature of its solidifying point or melting point or lower tosolidify it. The solution is only required to vaporize out a solventafter the impregnation to deposit a solute in a solid state. In asubstance capable of forming a solid substance by a chemical reaction,such as a polymerizable monomer, it is impregnated in the form of aliquid or solution, and a solid substance such as a solid polymer isthen formed by a chemical reaction such as a polymerization reaction.

The removal of the solid substance from the interior of the porousstructure is carried out by heating the porous resin base to atemperature exceeding the solidifying point or melting point of thesolid substance to melt it, thereby removing it as a liquid or bydissolving it with a solvent, thereby removing it as a solution. Theremoving method with the solvent may be called extraction.

When the liquid is that solidified by solidification or cooling, thesolidifying point or melting point thereof is preferably −150 to 150°C., more preferably −80 to 100° C. If the solidifying point or meltingpoint is too low, the cost of a cooling means for solidification becomesexpensive. If the solidifying point or melting point is too high, thetemperature comes near to the softening point or decomposition point ofthe porous resin base, so that there is a possibility deterioration ofthe porous resin base may be accelerated. In addition, if thesolidifying point or melting point is too high, such a substance becomeshighly viscous even when it is heated into a liquid, so that it isrequired to conduct vacuumization upon the impregnation to make theprocess complicated.

The liquid (substance) solidified by solidification or cooling may beany liquid so far as it can be solidified at a temperature not higherthan the softening point or decomposition point of the porous resin baseused, and preferably has a solidifying point or melting point within theabove-described range. Examples of such a liquid (substance) includewater, alcohols, hydrocarbons, polymers and mixtures of two or moresubstances thereof.

More specifically, examples of the impregnating liquid (substance)include water; alcohols such as methanol, ethanol, 1-propanol,2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol,2-methyl-2-propanol, 1-pentanol, cyclohexanol, 1-methylcyclohexanol,2-methylcyclohexanol, 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol,1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, glycerol and2-ethyl-(hydroxymethyl)-1,3-propanediol; and hydrocarbons such asbutane, pentane, n-hexane, 2,2-dimethylbutane, 2,3-dimethylbutane,heptane, n-octane, 2,2,3-trimethylpentane, isooctane, n-nonane,n-decane, n-dodecane, toluene, o-xylene, m-xylene, p-xylene,naphthalene, cyclopentane and cyclohexane.

As the impregnating liquid (substance), may also be used polymers in aliquid state at ordinary temperature, low-melting polymers in a solidstate at ordinary temperature, high-melting paraffins (alkanes; a sortof hydrocarbons) in a solid state at ordinary temperature, or the like.These polymers and paraffins may also be used in the form of a solution.

When a substance in a solid state at ordinary temperature is used as asolution, a solvent that can dissolve the substance in a solid state atordinary temperature, such as a polymer, paraffin or naphthalene, anddoes not dissolve or hardly dissolves the porous resin base is selectedas a solvent. The solvent preferably does not corrode, dissolve anddecompose the porous resin base.

The solution is preferably applied to a process in which the solution isimpregnated into the porous structure of the porous resin base bycasting or dipping, and the solvent is removed, thereby depositing asolid substance that is a solute. After the perforating, it is onlynecessary to dissolve the solid substance out of the porous structurewith the solvent used.

When a soluble polymer or high-melting paraffin is used as the liquid orsolution, the soluble polymer or paraffin can be used as a maskingmaterial upon making the inner wall surfaces of the perforationsconductive, in addition to the fact that perforating can be performedwith high precision.

In the first process according to the present invention for producing aporous resin base with the inner wall surfaces of perforations madeconductive, for example, a polymer material soluble in solvents or aparaffin in a solid state at ordinary temperature is used as a maskingmaterial. No particular limitation is imposed on the soluble polymer sofar as it is soluble in a solvent such as water or an organic solvent.However, it is preferably a polymer that has excellent affinity to theporous resin base, can be easily impregnated into the porous structureof the porous resin base.

The soluble polymer is such a polymer that the solvent dissolving thesoluble polymer can be easily penetrated into the porous structure ofthe porous resin base. The soluble polymer is preferably solid atordinary temperature (15 to 30° C.) in that perforations (through-holes)can be easily formed at ordinary temperature by a mechanical perforatingmethod.

For example, when a porous fluorocarbon resin sheet such as an expandedPTFE sheet is used as the porous resin base, an acrylic resin ispreferred as the soluble polymer. As examples of the acrylic resin, maybe mentioned homopolymers of alkyl esters of acrylic acid (i.e.,acrylate) or alkyl esters of methacrylic acid (i.e., methacrylates),such as polymethyl methacrylate (PMMA), and copolymers thereof.

As examples of the alkyl esters of acrylic acid and the alkyl esters ofmethacrylic acid, may be mentioned acrylates such as methyl acrylate,ethyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate,2-ethylhexyl acrylate, n-octyl acrylate, isooctyl acrylate, isononylacrylate, decyl acrylate and dodecyl acrylate; and methacrylates such asmethyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butylmethacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, n-octylmethacrylate, isooctyl methacrylate, isononyl methacrylate, decylmethacrylate, dodecyl methacrylate, cyclohexyl methacrylate, benzylmethacrylate and isobornyl methacrylate.

The soluble polymer may be an acrylic resin obtained by copolymerizingan alkyl ester of (meth)acrylic acid with another vinyl monomercopolymerizable therewith. As examples of another vinyl monomer, may bementioned carboxyl group-containing monomers such as acrylic acid,methacrylic acid, maleic acid, maleic anhydride and itaconic acid;(meth)acrylamide and derivatives thereof, such as acrylamide,methacrylamide and N-methylol-acrylamide; epoxy group-containingmonomers such as glycidyl(meth)acrylate; unsaturated nitriles such asacrylonitrile and methacrylonitrile; and vinyl aromatic compounds suchas styrene and p-methylstyrene. Another vinyl monomer is used in acopolymerization proportion of generally at most 30% by weight,preferably at most 20% by weight.

In the present invention, a paraffin (i.e., alkane) may be used as amasking material. As the paraffin, is preferably a paraffin in a solidstate at ordinary temperature from the viewpoint of easy formation ofthrough-holes at ordinary temperature. The melting point of the paraffinis preferably at least 15° C., more preferably at least 20° C.,particularly preferably at least 25° C. If the melting point of theparaffin is too low, it is necessary to lower the temperature of aworking environment or cool the composite sheet upon formation ofperforations by a machine-working method, so that such a paraffin is notdesirable from the viewpoint of energy cost.

As specific examples of the paraffin, may be mentioned hexadecane,heptadecane, octadecane, nonadecane, icosane, henicosane, docosane,triacontane and heptacontane. These paraffins may be used either singlyor in any combination thereof. The paraffin may be a mixture of two ormore compounds thereof. In such a case, any mixture may be well used sofar as the melting point of the mixture is preferably at least 15° C.even when the mixture contains a low-melting alkane. Likewise, theparaffin may contain impurities or the like mixed in upon the synthesisthereof. As the paraffin, may be used a commercially availablehigh-melting paraffin.

As the soluble polymer, is particularly preferred PMMA in that it isexcellent in affinity to the porous resin base such as the expandedPTFE, perforations can be easily formed by a machine-working method, itis not separated in a perforation-forming step and a catalyst-applyingstep, and it can be easily dissolved and removed by a solvent after itis used as the masking material.

In the present invention, as the liquid or solution impregnated into theporous structure of the porous resin base, may be used a liquid ofsolution containing a compound capable of forming a solid substance by achemical reaction. As the compound capable of forming a solid substanceby a chemical reaction, is typically a polymerizable monomer.

As the polymerizable monomer, is used a monofunctional monomer,preferably a monofunctional monomer having only one acryloyl group ormethacryloyl group. When a polyfunctional monomer having a bifunctionalor still higher polyfunctional group is used, a crosslinked structure isformed by a polymerization reaction, so that the solid substance formedbecomes insoluble or hardly soluble in solvents, and solvent extractioncannot be conducted. It is hence not preferable to use such apolyfunctional monomer.

No particular limitation is imposed on the monofunctional monomer so faras it can form a polymer soluble in solvents after a polymerizationreaction. As a specific example of the monofunctional monomer, may beused the acrylate or methacrylate used in forming the above-describedsoluble polymer. Among these monomers, methyl methacrylate, methylacrylate, isobonyl acrylate, isobonyl methacrylate and the like arepreferred.

Polymers formed from these polymerizable monomers are soluble in organicsolvents such as xylene, methyl ethyl ketone and acetone. Thesepolymerizable monomers may be used either singly or in any combinationthereof.

The polymerizable monomer is preferably low in viscosity and surfacetension in that it can be easily impregnated into the porous structureof the porous resin base. In this respect, methyl methacrylate isparticularly preferred. However, even a polymerizable monomer having ahigh viscosity may be impregnated into the porous resin base by loweringits viscosity by heating upon the impregnation into the porous resinbase. A polymerizable monomer having a high surface tension may beimpregnated into the porous resin base by lowering its surface tensionby addition of a surfactant.

When perforations are formed by machine working, the polymerizablemonomer is preferably such that it can form a hard and brittle polymerfrom the viewpoint of preventing occurrence of burr. The polymer formedpreferably has a high glass transition temperature in that it is notsoftened even when the temperature of worked sites is raised bygeneration of frictional heat when the perforations are formed bymachine working. In this regard, isobonyl methacrylate, which forms apolymer having a glass transition temperature as high as 180° C., ispreferred.

When a solution (hereinafter may be referred to as “polymerizablemonomer solution”) obtained by dissolving a polymer obtained bypolymerizing a polymerizable monomer in advance is used in thepolymerizable monomer, volume shrinkage occurred at the time thepolymerizable monomer has been polymerized can be inhibited, wherebywarpage and distortion of the porous resin base can be inhibited, and inturn, the perforations can be formed with good precision. It is hencepreferable to use such a polymerizable monomer solution. Theconcentration of the polymer may be suitably selected within a rangethat the viscosity of the monomer solution does not become very high.However, it is generally at most 50% by weight, preferably at most 30%by weight. The number average molecular weight of the polymer ispreferably 10,000 to 1,000,000. If the molecular weight of the polymeris too low, the effect to inhibit the volume shrinkage becomes small. Ifthe molecular weight is too high, the solubility of such a polymer inthe monomer becomes low.

Methods for polymerizing the polymerizable monomer include heatpolymerization and photopolymerization methods. In order to shortenoperating time, the photopolymerization method is preferably adopted.When the thickness of the porous resin base is great, light is hard tobe transmitted. In such a case, the heat polymerization method ispreferably used.

In the case of the photopolymerization, a photopolymerization initiatoris added to a polymerizable monomer or a polymerizable monomer solution.A proportion of the photopolymerization initiator added is generally 0.1to 5% by weight based on the whole weight of the monomer. examples ofthe photopolymerization initiator include benzophenone and thioxanthoneas those of a hydrogen abstraction type, and α-aminoalkylphenone,α-hydroxyalkyl-phenone and acylphosphine oxide as those ofintramolecular cleavage.

In the case of heat polymerization, an azo compound such asazoisobutyronitrile or a peroxide such as dicumyl peroxide is added as aheat polymerization initiator to a polymerizable monomer or apolymerizable monomer solution. A proportion of the heat polymerizationinitiator added is generally 0.1 to 5% by weight based on the wholeweight of the monomer.

Besides the polymerization initiator, additives such as a surfactant, anantioxidant, a photosensitizer, a lubricant and a parting agent may beadded to the polymerizable monomer or polymerizable monomer solution asneeded.

As a method for impregnating the porous structure of the porous resinbase with the polymerizable monomer, may be used casting or dipping.After the impregnation, light irradiation or heating is conductedaccording to the kind of the polymerization initiator added to thepolymerizable monomer to conduct polymerization, thereby forming apolymer.

3. Production Process of Perforated Porous Resin Base

In the present invention, a perforated porous resin base is produced aseries of steps comprising the following Steps 1 to 4:

(1) Step 1 of impregnating the porous structure of a porous resin basewith a liquid or solution;

(2) Step 2 of forming a solid substance from the liquid or solutionimpregnated;

(3) Step 3 of forming a plurality of perforations extending through fromthe first surface of the porous resin base having the solid substancewithin the porous structure to the second surface in the porous resinbase; and

(4) Step 4 of melting or dissolving the solid substance to remove itfrom the interior of the porous structure.

As described above, the process according to the present invention has afeature in that the porous structure of the porous resin base isimpregnated with the liquid or solution, the solid substance is formedfrom the liquid or solution impregnated, and the perforations are thenformed.

A machine-working method can be applied to the perforating. Since thesolid substance is filled into the porous structure of the porous resinbase, the perforating can be carried out in the same manner as in anonporous resin base.

As the porous resin base, is generally used a porous resin sheet. As theporous resin sheet is preferred a porous fluorocarbon resin sheet, withan expanded PTFE being more preferred. The expanded PTFE sheet has, as aporous structure, a microstructure comprising fibrils and nodesconnected to each other by the fibrils.

In Step 1, the liquid or solution is preferably impregnated into theporous structure of the porous resin base by casting or dipping. As theliquid used, is the above-described substance having a solidifying pointor melting point within a range of from −150 to 150° C. As such asubstance is preferred water, an alcohol, a hydrocarbon, a polymer or amixture of two or more substances thereof. The substance having thesolidifying point or melting point within the range of from −150 to 150°C. is preferably a paraffin having a melting point of at least 15° C.

In Step 1, the substance (water, alcohol, hydrocarbon or polymer) havingthe solidifying point or melting point within the range of from −150 to150° C. is impregnated as a liquid of a temperature exceeding thesolidifying point or melting point into the porous structure of theporous resin base. In Step 2, the substance is solidified at atemperature not higher than the solidifying point or melting point toform a solid substance.

In Step 3, perforations are formed in a state that the solid substanceexists within the porous structure. In Step 4 after the perforatingstep, the substance is melted at a temperature exceeding the solidifyingpoint or melting point to remove it. When the impregnating substance hasa high melting point, a solvent is used to dissolve out it.

As the solution used in Step 1, may be used a solution of a solublepolymer or paraffin having a melting point of 15 to 30° C. When such asolution is used, the solution of the soluble polymer or paraffin isimpregnated into the porous structure of the porous resin base in Step1, the solvent is vaporized out in Step 2 to form a solid substance ofthe polymer or paraffin. In Step 3, perforations are formed in a statethat the solid substance exists within the porous structure. In Step 4after the perforating step, the solid substance is dissolved with asolvent to remove it.

In Step 1, a liquid or solution containing a compound capable of forminga solid substance by a chemical reaction may be used as the liquid orsolution. The compound capable of forming a solid substance by achemical reaction is preferably a polymerizable monomer undergoing apolymerization reaction by heat or light to form a polymer. As theliquid or solution containing the compound capable of forming a solidsubstance by a chemical reaction, may be used a polymerizable monomersolution also containing, in addition to a polymerizable monomer, apolymer obtained by the polymerization of the polymerizable monomer. Asthe polymerizable monomer, is preferred a monofunctional acrylate ormethacrylate.

When the polymerizable monomer or the like is used, the liquid orsolution containing the compound capable of forming a solid substance bya chemical reaction is impregnated into the porous structure of theporous resin base in Step 1, and the compound is subjected to thechemical reaction in Step 2 to form a solid substance. In Step 3,perforations are formed in a state that the solid substance existswithin the porous structure. In Step 4 after the perforating step, thesolid substance is dissolved with a solvent to remove it.

In the perforating Step 3, perforations are formed by means of, forexample, i) a mechanically perforating method, ii) a method of etchingby a light-abrasion method, or iii) a method of perforating by using anultrasonic head equipped with at least one oscillator on the tip thereofand pressing the tip of the oscillator against the porous resin base toapply ultrasonic energy thereto.

The first surface and second surface of the porous resin base meanportions having a large surface area. For example, when the porous resinbase is a sheet, the first surface or second surface means a surface(the so-called front or back surface of the sheet) of a wide flatportion, not a surface of a portion having a small thickness.

As the perforating method, is preferred the mechanically perforatingmethod or the method (method of perforating by ultrasonically vibratingthe tip of a tool) of perforating by using the ultrasonic head equippedwith at least one oscillator on the tip thereof and pressing the tip ofthe oscillator against the porous resin base to apply ultrasonic energythereto. As the mechanically perforating method (machine-workingmethod), is preferred punching, blanking or drill. By ultrasonicvibration, a working speed is increased, and workability is improved.

A porous resin base rich in flexibility and elasticity, such as sponge,may be generally subjected to machine working with good precision at atemperature not higher than the first-order transition point of theresin or a temperature near to the first-order transition point likerubber materials. However, a porous resin material containing a greatnumber of porous structures is difficult to retain its form at atemperature not higher than the first-order transition point thereof dueto its friability in the vicinity of performed portions upon the machineworking.

The present invention has a feature in that a state that the solidsubstance has been filled into the porous structure of the porous resinbase is created, whereby mechanical perforating can be easily conducted.One of other excellent features of the present invention resides in thata substance having a solidifying point or melting point suited to thepurpose of use is selected, whereby a working temperature can beselected irrespective of the first-order transition point of a resinforming the porous resin base. When a polymeric substance capable ofbeing easily removed later on is filled into the porous structure of theporous resin base, the machine working can be easily conducted likewise.

When the porous resin base is a continuous sheet, for example, theimpregnating step, the cooling step and the perforating step by themachine working are arranged in this order, and these steps aresuccessively conducted, whereby the products can be continuouslyobtained. After the perforating step, the temperature is raised to atemperature higher than the solidifying point or melting point, wherebythe solid substance can be removed as a liquid.

When the solution such as the polymer solution is used, it is onlynecessary to arrange a step of drying and removing the solvent in placeof the cooling step. After the perforating step, a step of dissolvingthe solid substance with a solvent to remove it is arranged.

The perforated porous resin base obtained by the production processaccording to the present invention has perforations (through-holes) at aplurality of positions predetermined according to its use. Theperforated porous resin base is free of deformation due to theperforating, collapse of the porous structure at peripheral portions ofthe perforations, including the inner wall surfaces thereof, andoccurrence of burr, and the perforations have sharp edges.

FIG. 1 is a microphotograph of a perforated portion in an expanded PTFEsheet that was perforated in Example 1 of the present invention, and itcan be seen that the perforations having sharp edges are formed. On theother hand, FIG. 2 is a microphotograph of a perforated portion in anexpanded PTFE sheet that was perforated in Comparative Example 3, inwhich occurrence of burr and partial collapse of the porous structureare observed.

Since the porous structure is also retained on the inner wall surfacesof the perforations in the perforated porous resin base obtained by theproduction process according to the present invention, it may be appliedto a separation membrane. When a conductive metal is applied to theinner wall surfaces of the perforations, it can be utilized as amaterial for connection between circuits or an anisotropicallyconductive material. When the porous resin base is an expanded PTFE basehaving a microstructure comprising fibrils and nodes connected to eachother by the fibrils, it is rich in flexibility and elasticity, low indielectric constant and particularly excellent in electricallyinsulating property because the porous structure is retained. Theexpanded PTFE base is also very useful as a medical device because it isexcellent in chemical stability.

4. Production Process (1) of Porous Resin Base with the Inner WallSurfaces of Perforations Made Conductive

A process according to the present invention for producing a porousresin base (for example, an anisotropically conductive sheet) with theinner wall surfaces of perforations selectively made conductive is aprocess for selectively making the inner wall surfaces of theperforations conductive by a series of steps comprising the followingSteps I to VI:

(1) Step I of impregnating the porous structure of a porous resin baseincluding both surfaces thereof with a soluble polymer or paraffin, or acompound capable of forming a solid substance by a chemical reaction;

(2) Step II of forming a solid substance from the soluble polymer orparaffin, or the compound capable of forming a solid substance by achemical reaction, which has been impregnated, to form a composite sheetof a structure that both surfaces of the porous resin base have a layerof the solid substance, and the solid substance is impregnated into theporous structure;

(3) Step III of forming a plurality of perforations extending throughfrom the first surface of the composite sheet to the second surface inthe composite sheet;

(4) Step IV of applying a catalyst facilitating a reducing reaction of ametal ion to the surfaces of the composite sheet including the innerwall surfaces of the respective perforations;

(5) Step V of removing the solid substance from the composite sheet; and

(6) Step VI of using the catalyst applied to and remaining on the innerwall surfaces of the respective perforations in the porous resin base toapply a conductive metal to the inner wall surfaces.

In Step I, the soluble polymer or paraffin, or the compound capable offorming a solid substance by a chemical reaction is impregnated into theporous structure of the electrically insulating porous resin base(ordinarily, porous resin sheet) including both surfaces thereof.

In Step II, the solid substance is formed from the soluble polymer orparaffin, or the compound capable of forming a solid substance by achemical reaction, which has been impregnated, to form a composite sheetof the structure that both surfaces of the porous resin base have alayer of the solid substance, and the solid substance is impregnatedinto the porous structure.

Processes for forming the composite sheet include a process comprisingimpregnating the soluble polymer or paraffin by a method of casting asolution or melt of the soluble polymer or paraffin on both surfaces ofthe porous resin base or dipping the porous resin base in a solution ormelt of the soluble polymer or paraffin in Step I, and forming acomposite sheet of the structure that both surfaces of the porous resinbase have a solid layer of the soluble polymer or paraffin, and thesolid soluble polymer or paraffin is impregnated into the porousstructure by a method of vaporizing out the solvent or lowering thetemperature of the porous resin base to a temperature not higher thanthe solidifying point or melting point of the soluble polymer orparaffin in Step II.

Other processes for forming the composite sheet include a processcomprising impregnating the compound capable of forming a solidsubstance by a chemical reaction by a method of casting a liquid orsolution containing such a compound on both surfaces of the porous resinbase or dipping the porous resin base in a liquid or solution containingthe compound in Step I, and forming a composite sheet of the structurethat both surfaces of the porous resin base have a layer of the solidsubstance, and the solid substance is impregnated into the porousstructure by a method of forming the solid substance by the chemicalreaction in Step II.

In the case of the latter process, is preferably adopted a processcomprising, in Step I, impregnating a polymerizable monomer undergoing apolymerization reaction by heat or light to form a polymer by a methodof casting a liquid or solution containing the polymerizable monomer onboth surfaces of the porous resin base or dipping the porous resin basein a liquid or solution containing the polymerizable monomer, and inStep II, forming a composite sheet of the structure that both surfacesof the porous resin base have a solid polymer layer, and a solid polymeris impregnated into the porous structure by a method of polymerizing thepolymerizable monomer by heat or light to form the solid polymer.

When casting is conducted on the both surfaces of the porous syntheticresin sheet with a solution of the soluble resin or paraffin, or such asolution is impregnated into voids in the porous structure, for example,a ketone such as acetone or methyl ethyl ketone; an ester such as ethylacetate or butyl acetate; a halogenated hydrocarbon such asdichloroethane or dichloromethane; an aromatic hydrocarbon such asxylene or toluene; a polar organic solvent such as tetrahydrofuran,chloroform, diacetone alcohol or dimethylformamide; or the like may beused as a solvent.

The solvent may be suitably selected according to the kinds of thesoluble polymer, paraffin and porous resin base. For example, when anexpanded PTFE sheet is used as the porous resin base, and polymethylmethacrylate (PMMA) is used as the soluble polymer, a polar solvent thatcan dissolve PMMA and is easy to penetrate into the porous structure ofthe expanded PTFE sheet, such as acetone or tetrahydrofuran, ispreferably used as the solvent.

When the melting point of the soluble polymer is low, and so it can bemelted by heating to a temperature of, preferably, 100° C. or lower, themelt thereof may be used to conduct casting and impregnation. When theparaffin also has not so high melting point, and the melting point ispreferably 100° C. or lower, a melt obtained by heating it to atemperature not lower than the melting point can be used to conductcasting and impregnation.

When as a production process of the composite sheet, for example, anexpanded PTFE sheet is used as the porous synthetic resin sheet, andPMMA is used as the masking material to impregnate it up to the interiorof the porous structure, it is preferable to use a solution with PMMAdissolved at a concentration of about 10 to 30% by weight in a polarsolvent that can dissolve PMMA and is easy to penetrate into the porousstructure of the expanded PTFE sheet, such as acetone ortetrahydrofuran. It is only necessary to slowly dip the expanded PTFEsheet in this solution so as not leave air within the porous structureto impregnate it. According to the dipping method, the interior of theporous structure of the porous resin base can be filled with the solublepolymer, and at the same time the surface portion thereof can also becovered with the soluble polymer. This soluble polymer layer fulfills afunction as a mask layer. Even when the high-melting paraffin is used,both surfaces of the porous resin base can be covered with the paraffinlikewise to form mask layers.

When the expanded PTFE sheet is used as the porous resin base, PMMA isused as the masking material, and PMMA films are formed as mask layerson both surfaces of the expanded PTFE sheet, it is preferable to adopt aprocess comprising casting a solution with PMMA dissolved at aconcentration of about 10 to 40% by weight in acetone, tetrahydrofuranor the like in the same manner as described above on both surfaces ofthe expanded PTFE sheet. In this case, it is preferable to conduct thecasting while heating the expanded PTFE sheet to a temperature of about30 to 60° C. to facilitate the vaporization of the solvent. According tothe casting method, the soluble polymer is penetrated into not only thesurface portion of the porous resin base, but also the interior of theporous structure under the surface portion.

When the paraffin is used, it is preferable to adopt a processcomprising heating and melting the paraffin in a solid state at ordinarytemperature and dipping the expanded PTFE sheet in the melt thusobtained to fill the interior of the porous structure with the paraffin.

When the interior of the porous structure of the porous resin base isfilled with the soluble polymer or paraffin, collapse of the porousstructure in the vicinity of worked portions, i.e., perforated portionscan be prevented even when perforations are formed by a machine-workingmethod. When the interior of the porous structure of the porous resinbase is filled with the soluble polymer or paraffin, the porousstructure can be sufficiently retained upon the formation of theperforations. However, the application of the catalyst is limited to thesurface portions of the inner wall surfaces of the perforations.

On the other hand, when the mask layer is formed only in the vicinity ofthe surface of the porous resin base, the performance of retaining theporous structure in the vicinity of the perforations is lowered when theperforations are formed by a machine-working method. However, thecatalyst can be applied up to a depth of about several microns from thesurface of the inner wall surface in the step of applying the catalystto the inner wall surfaces of the perforations though it variesaccording to the porosity of the porous resin base. When the solublepolymer or paraffin is impregnated into the porous structure of theporous resin base by the casting or dipping method, thus the degree ofimpregnation is controlled to form mask layers on both surfaces and toreduce the amount impregnated into the porous structure, whereby thecatalyst can be sufficiently applied to the resin portion of the innerwall surfaces of the perforations after the perforating step.

In the present invention, a plurality of perforations (through-holes)extending through from the first surface of the composite sheet to thesecond surface are formed. Examples of methods for forming theperforations include i) a mechanically perforating method, ii) a methodof etching by a light-abrasion method, and iii) a method of perforatingby using an ultrasonic head equipped with at least one oscillator on thetip thereof and pressing the tip of the oscillator against the compositesheet to apply ultrasonic energy thereto.

In order to mechanically perforate, there may be adopted amachine-working method, for example, pressing, punching or drillingmethod. According to the machine-working method, through-holes having arelatively large hole diameter of generally at least 100 μm, often atleast 300 μm can be formed cheaply.

In order to form the through-holes by irradiation of laser beams, it ispreferable to adopt a method comprising irradiate the surface of thecomposite sheet with laser beams through a light-screening sheet havinga plurality of light-transmitting portions (openings) independent of oneanother in accordance with a predetermined pattern, thereby formingpatterned through-holes. The portions irradiated with the beamstransmitted through the plurality of the openings in the light-screeningsheet are etched to form the through-holes. According to this method,through-holes having a small hole diameter can be formed.

In the ultrasonic method, the ultrasonic head equipped with at least oneoscillator on the tip thereof is used to apply ultrasonic energy to thecomposite sheet, thereby forming patterned perforations in the compositesheet. The ultrasonic energy is applied only in the vicinity of theportions of the composite sheet, with which the tip of the oscillatorcomes into contact, to locally raise the temperature of the sheet byvibratory energy owing to ultrasonic waves, thereby easily cutting andremoving the resin to form the perforations.

The form of the perforations (through-holes) may be any of circular,elliptical, star, octagonal, hexagonal, rectangular and triangularforms. The hole diameter of the perforations can be controlled togenerally about 5 to 100 μm, preferably about 5 to 30 μm in applicationfields that a small hole diameter suits, while it can be controlled togenerally about 100 to 3,000 μm, preferably about 150 to 2,000 μm, morepreferably 200 to 1,500 μm in application fields that a relatively largehole diameter suits. The plurality of the perforations is preferablyformed in a predetermined pattern corresponding to the distribution ofelectrodes of a circuit devices or the like.

When the perforations are formed in the composite sheet, the resinportion of the porous structure is exposed to the inner wall surfaces ofthe perforations. In the case of the expanded PTFE sheet, the resinportion of the porous structure is composed of fibrils and nodes formedfrom PTFE.

In the present invention, a catalyst facilitating a reducing reaction ofa metal ion is applied to the surfaces of the composite sheet includingthe inner wall surfaces of the respective perforations. As a method forapplying a conductive metal to the inner wall surfaces of theperforations in the porous resin base, is preferred an electrolessplating method. In the electroless plating method, a catalystfacilitating a chemically reducing reaction is generally applied toportions to deposit a plated metal in advance. In order to conductelectroless plating only on the inner wall surfaces of the perforationsin the porous resin base, it is necessary to apply the catalyst only tosuch portions. When a plated metal is applied to other portions than theinner wall surfaces of the perforations, there is a possibility thatrespective conductive portions formed by a conducted metal applied tothe inner wall surfaces of the respective perforations mayshort-circuit.

When the plurality of the perforations are formed in the compositesheet, and the catalyst is applied to the surfaces of the compositesheet including the inner wall surfaces of the respective perforations,the catalyst is also applied to the resin portion of the porousstructure exposed to the inner wall surfaces of the perforations in theporous resin base.

In order to apply the catalyst facilitating the chemically reducingreaction of a metal ion, it is only necessary to dip the compositesheet, in which the perforations have been formed, in, for example, apalladium-tin colloid catalyst-applying liquid while sufficientlystirring the liquid.

In the present invention, the soluble polymer or paraffin is removedfrom the composite sheet after the above-described step. The solublepolymer layers or paraffin layers remaining on both surfaces of theporous resin base may be released to remove them. However, it ispreferable to adopt a method of dissolving them together with thesoluble polymer or paraffin impregnated into the porous structure usinga solvent.

No particular limitation is imposed on the solvent used in thedissolution and removal of the soluble polymer or paraffin so far as itcan dissolve the soluble polymer or paraffin. However, it is preferablya solvent that does not dissolve or hardly dissolves the porous resinbase. When the expanded PTFE sheet is used as the porous resin base, andPMMA is used as the masking material, it is preferable to use, as thesolvent, a polar solvent such as acetone or tetrahydrofuran. In the caseof the paraffin, it can be dissolved and removed with acetone or thelike. The dissolution and removal of the soluble polymer or paraffin isconducted by a method of dipping the composite sheet in the solvent.

When the soluble polymer or paraffin is removed from the compositesheet, the catalyst applied to the inner wall surfaces of theperforations in the porous resin base remains.

In the present invention, the catalyst applied to remaining on the innerwall surfaces of the perforations (through-holes) in the porous resinbase is utilized to apply the conductive metal to the inner wallsurfaces. As a method for applying the conductive metal, is suitablyadopted an electroless plating method.

The catalyst (for example, palladium-tin) remaining on the inner wallsurfaces of the perforations is activated prior to the electrolessplating. More specifically, the porous resin base is dipped in anorganic acid salt or the like, which is commercially available foractivating plating catalysts, thereby dissolving tin to activate thecatalyst.

The porous resin base, to the inner wall surfaces of the perforations inwhich the catalyst has been applied, is dipped in an electroless platingsolution, whereby the conductive metal can be deposited only to theinner wall surfaces of the perforations, thereby forming cylindricalconductive portions (also referred to as conductive paths orelectrodes). Examples of the conductive metal include copper, nickel,silver, gold and nickel alloys. When particularly high conductivity isrequired, copper is preferably used.

When the expanded PTFE sheet is used, plating particles (crystal grains)are first deposited so as to be entangled in fibrils exposed to theinner wall surfaces of the perforations in the porous PTFE sheet, sothat the condition of the conductive metal applied can be controlled bycontrolling the time of the plating. If the time of the electrolessplating is too short, it is difficult to achieve conductivity in thethickness-wise direction of the sheet. If the time of the electrolessplating is too long, the conductive metal becomes a metal mass, so thatit is difficult to elastically recover the sheet under a compressiveload ordinarily used. The plating is conducted in a moderate amount,whereby the conductive metal layers can be formed in the state that theporous structure has been retained, so that it is possible to impart theconductivity in the thickness-wise direction together with theelasticity.

The thickness (for example, the thickness of fibrils of the expandedPTFE sheet) of the resin portion of the porous structure is preferablyat most 50 μm. The particle diameter of the conductive metal ispreferably about 0.001 to 5 μm. The amount of the conductive metalapplied is preferably controlled to about 0.01 to 4.0 g/ml for thepurpose of retaining the porous structure and elasticity.

In order to improve the prevention of oxidation and electrical contactproperty, the cylindrical conductive portions formed in theabove-described process preferably makes use of an antioxidant or iscoated with a noble metal or noble metal alloy. As the noble metal,palladium, rhodium or gold is preferred in that it has a low electricresistance. The thickness of the coating layer formed of the noble metalor the like is preferably 0.005 to 0.5 μm, more preferably 0.01 to 0.1μm. If the thickness of this coating layer is too small, the effect toimprove the electrical contact property becomes small. If the thicknessis too great, the coating layer becomes liable to be separated. It ishence not preferable to coat the conductive portions in such a too smallor too great thickness. When the conductive portions are coated with,for example, gold, a method of coating the conductive metal layers withnickel of about 8 nm and then conducting displacement plating with goldis effective.

According to the production process of the present invention, theperforations extending through from the first surface to the secondsurface can be formed at a plurality of positions of the porous resinbase. In addition, an anisotropically conductive sheet that hasconductive portions formed by the conductive metal applied to the resinportion of the porous structure in the inner wall surfaces of theperforations and permits imparting conductivity only to thethickness-wise direction of the sheet by the conductive portions can beproduced.

While referring to the drawing, the above-described production processis described taking the case of using the soluble polymer or paraffin asan example. FIG. 3 is a flow diagram illustrating an exemplaryproduction process according to the present invention. As illustrated inFIG. 3(a), a porous resin base (for example, expanded PTFE sheet) 1 isprovided. As illustrated in FIG. 3(b), the interior of the porousstructure, including both surfaces thereof, is impregnated with asoluble polymer 2 to prepare a composite sheet 3, which has solublepolymer layers (coating layers) on both surfaces and in which thesoluble polymer has been impregnated into the porous structure. Asillustrated in FIG. 3(c), the composite sheet is then perforated toproduce a composite sheet, in which a plurality of perforations 4, 4have been formed.

As illustrated in FIG. 3(d), a plating catalyst 5 is applied to thesurfaces of the composite sheet including the inner wall surfaces of theperforations. As illustrated in FIG. 3(e), the soluble polymer is thendissolved and removed, whereby the plating catalyst applied to thesurfaces of the soluble polymer layers (mask layers) on both surfaces ofthe porous resin base is removed together with the soluble polymer, andonly the catalyst applied to the inner wall surfaces of the perforationsin the porous resin base remains. FIG. 3(f) illustrates an electrolessplating step. When the electroless plating is conducted, platingparticles (conductive metal particles) are deposited only on the innerwall surfaces of the perforations, to which the catalyst has beenapplied, to form conductive metal layers 6. A porous resin base 7 withonly the inner wall surfaces of the perforations selectively madeconducted can be obtained in such a manner.

In the above-described production process, a porous resin sheet ispreferably used as the porous resin base. As the porous resin sheet, ispreferred a porous fluorocarbon resin sheet, with an expanded PTFE sheethaving, as a porous structure, a microstructure comprising fibrils andnodes connected to each other by the fibrils being more preferred. Thesoluble polymer or paraffin is preferably in a solid state at atemperature within a range of 15 to 30° C.

In Step I, a solution or melt of the soluble polymer or paraffin is caston both surfaces of the porous resin base, or the porous resin base isdipped in the solution or melt of the soluble polymer or paraffin. InStep II, the composite sheet of the structure that both surfaces of theporous resin base have a solid layer of the soluble polymer or paraffin,and the solid soluble polymer or paraffin is impregnated into the porousstructure is preferably formed by a method of vaporizing out the solventor lowering the temperature of the porous resin base to a temperaturenot higher than the solidifying point or melting point of the solublepolymer or paraffin.

In Step III, a plurality of the perforations are preferably formed inthe composite sheet by i) a mechanically perforating method, ii) amethod of etching by a light-abrasion method, or iii) a method ofperforating by using an ultrasonic head equipped with at least oneoscillator on the tip thereof and pressing the tip of the oscillatoragainst the composite sheet to apply ultrasonic energy thereto.

In Step V, the soluble polymer or paraffin is preferably dissolved andremoved by using a solvent that does not dissolve or hardly dissolvesthe porous resin base, but exhibits good solubility for the solublepolymer or paraffin. In Step V, the soluble polymer or paraffin may alsobe melted and removed.

In Step VI, the conductive metal is preferably applied to the inner wallsurfaces of the respective perforations by electroless plating.According to the production process of the present invention, there canbe produced an anisotropically conductive sheet, in which the porousresin base with the inner wall surfaces of the perforations madeconductive has conductive portions formed by the conductive metalapplied to the resin portion of the porous structure in the inner wallsurfaces of the plurality of the perforations extending through from thefirst surface to the second surface, and which permits impartingconductivity only to the thickness-wise direction of the sheet by theconductive portions.

With respect to the above-described production process, the case wherethe soluble polymer or paraffin is used has been mainly described.However, a compound capable of forming a solid substance by a chemicalreaction, such as a polymerizable monomer, may also be used in place ofsuch a substance to form mask layers by the solid substance likewise,thereby producing a porous resin base (anisotropically conductive sheet)with only the inner wall surfaces of perforations made conductive.

5. Production Process (2) of Porous Resin Base with the Inner WallSurfaces of Perforations Made Conductive

Another process according to the present invention for producing aporous resin base (for example, an anisotropically conductive sheet)with the inner wall surfaces of perforations selectively made conductiveis a process for selectively making the inner wall surfaces of theperforations conductive by a series of steps comprising the followingSteps i to viii:

(1) Step i of laminating, as mask layers, porous resin layers (B) and(C) on both surfaces of a porous resin base (A) to form a laminate of a3-layer structure;

(2) Step ii of impregnating the respective porous structures of thelaminate with a soluble polymer or paraffin, or a compound capable offorming a solid substance by a chemical reaction;

(3) Step iii of forming a solid substance from the soluble polymer orparaffin, or the compound capable of forming a solid substance by achemical reaction, which has been impregnated;

(4) Step iv of forming a plurality of perforations extending throughfrom the first surface of the laminate having the solid substance withinthe respective porous structures to the second surface in the laminate;

(5) Step v of dissolving the solid substance to remove it from theinteriors of the respective porous structures;

(6) Step vi of applying a catalyst facilitating a reducing reaction of ametal ion to the surfaces of the laminate including the inner wallsurfaces of the respective perforations;

(7) Step vii of removing the mask layers from both surfaces of theporous resin base (A); and

(8) Step viii of using the catalyst applied to and remaining on theinner wall surfaces of the respective perforations in the porous resinbase (A) to apply a conductive metal to the inner wall surfaces.

In the above-described process, the solid polymer or paraffin ispreferably formed by a process comprising impregnating the solublepolymer or paraffin by casting a solution or melt of the soluble polymeror paraffin on both surfaces of the laminate or dipping the laminate ina solution or melt of the soluble polymer or paraffin in Step ii, andvaporizing out the solvent or lowering the temperature of the laminateto a temperature not higher than the solidifying point or melting pointof the soluble polymer or paraffin in Step iii.

In the above process, the solid polymer or paraffin is also preferablyformed by impregnating a liquid or solution containing, as the compoundcapable of forming a solid substance by a chemical reaction, apolymerizable monomer undergoing a polymerization reaction by heat orlight to form a polymer into the respective porous structures of thelaminate in Step ii, and polymerizing the polymerizable monomer by heator light in Step iii.

While referring to the drawing, the above-described production processis described taking the case of using the soluble polymer as an example.FIG. 4 is a flow diagram illustrating the respective steps adopted inthe production process of the present invention. As illustrated in FIGS.4(A) and 4(B), porous resin layers 42 and 43 are laminated as masklayers on both surfaces of a porous resin base 41 to form a laminate 44of a 3-layer structure. The porous resin layers, which will become masklayers, may be the same or different from the porous resin base. Thesame 3 porous resin bases are generally used to form the laminate.

In order to effectively mask both surfaces of the porous resin base 41,it is preferable to fusion bond the respective layers to each other tointegrate them. When expanded PTFE sheets are used as the porous resinbase and mask layers, the respective layers are easily fusion bonded toeach other and integrated by heating and pressure welding them, and themask layers can be easily separated if necessary.

As illustrated in FIG. 4(C), a liquid or solution containing thecompound capable of forming a solid substance by a chemical reactioninto the respective porous structures of the laminate 44. As thecompound capable of forming a solid substance by a chemical reaction, ispreferred a polymerizable monomer undergoing a polymerization reactionby heat or light to form a polymer. The liquid or solution containingthe compound capable of forming a solid substance by a chemical reactionmay be a solution also containing, in addition to a polymerizablemonomer, a polymer obtained by the polymerization of the polymerizablemonomer.

As the polymerizable monomer, is preferred the above-describedmonofunctional acrylate or methacrylate such as methyl methacrylate. Aphotopolymerization initiator or heat polymerization initiator is addedto the polymerizable monomer.

As illustrated in FIG. 4(D), the compound in the liquid or solutionimpregnated is subjected to a chemical reaction to form a solidsubstance. In this step, the polymerizable monomer is subjected tophotopolymerization or heat polymerization to form a polymer (forexample, PMMA) in a solid state at ordinary temperature. In such amanner, a laminate 46 in which all the 3 layers have been filled withthe polymer is obtained.

As illustrated in FIG. 4(E), a plurality of perforations 48 extendingthrough from the first surface of the laminate having the solidsubstance within the respective porous structures to the second surfaceare then formed in the laminate. As illustrated in FIG. 4(F), the solidsubstance (polymer) is dissolved with a solvent to remove it from theinteriors of the respective porous structures in the laminate 47 inwhich the perforation have been formed.

As illustrated in FIG. 4(G), a catalyst (plating catalyst) facilitatinga reducing reaction of a metal ion is applied to the surfaces of thelaminate 49 subjected to the solvent extraction, including the innerwall surfaces of the respective perforations. At this time, the porousresin layers 42 and 43 arranged on both surfaces function as respectivemask layers to prevent the catalyst from being applied to both surfacesof the porous resin base 41.

As illustrated in FIG. 4(H), the mask layers 42 and 43 are then removedfrom the laminate 50, to which the plating catalyst has been applied, toobtain a porous resin base 51 with the plating catalyst applied only tothe inner wall surfaces of the perforations. As illustrated in FIG.4(I), the catalyst applied to and remaining on the inner wall surfacesof the respective perforations in the porous resin base 51 is used toapply a conductive metal to the inner wall surfaces. The application ofthe conductive metal is generally conducted by an electroless platingmethod. In such a manner, a porous resin base 52 with only the innerwall surfaces of the perforations made conductive can be obtained.

As the porous resin base, is preferred a porous resin sheet. As theporous resin sheet, is preferred a porous fluorocarbon resin sheet, withan expanded PTFE sheet, with an expanded PTFE sheet having, as a porousstructure, a microstructure comprising fibrils and nodes connected toeach other by the fibrils being more preferred.

In Step iv, a plurality of the perforations are preferably formed in thelaminate by i) a mechanically perforating method, ii) a method ofetching by a light-abrasion method, or iii) a method of perforating byusing an ultrasonic head equipped with at least one oscillator on thetip thereof and pressing the tip of the oscillator against the laminateto apply ultrasonic energy thereto.

In Step v, the solid substance is preferably dissolved and removed byusing a solvent that does not dissolve or hardly dissolves the porousresin base, but exhibits good solubility for the solid substance.

In Step vi, the catalyst (plating catalyst) facilitating the reducingreaction of the metal ion is applied to the surfaces of the laminateincluding the inner wall surfaces of the respective perforations. Inthis production process, the solid substance has been dissolved andremoved from the interior of the porous structure in Step v prior to theapplication of the plating catalyst, so that the resin portion (forexample, fibrils of PTFE) of the inner wall surfaces of the perforationsis sufficiently exposed. Therefore, the plating catalyst can be firmlyapplied up to the resin portion (resin portion located at a depth ofabout several microns from the surface of the inner wall surface)located at a somewhat interior position of the porous structure of theinner wall surfaces of the perforations. In Step viii, the conductivemetal is preferably applied to the inner wall surfaces of the respectiveperforations by electroless plating.

According to the production process of the present invention, there canbe produced an anisotropically conductive sheet, in which the porousresin base with the inner wall surfaces of the perforations madeconductive has conductive portions formed by the conductive metalapplied to the resin portion of the porous structure in the inner wallsurfaces of the plurality of the perforations extending through from thefirst surface to the second surface, and which permits impartingconductivity only to the thickness-wise direction of the sheet by theconductive portions.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples. However, the presentinvention is not limited to these examples only. Physical propertieswere measured in accordance with the following respective methods.

(1) Bubble Point (BP):

A bubble point of a porous PTFE film by an expanding process wasmeasured in accordance with ASTM-F-316-76 using isopropyl alcohol.

(2) Porosity:

A porosity of a porous PTFE film by an expanding process was measured inaccordance with ASTM D-792.

(3) Conduction-Starting Load:

A conduction-starting load of an anisotropically conductive film wasmeasured by means of a testing equipment illustrated in FIG. 5. In thetesting equipment illustrated in FIG. 5, an anisotropically conductivesheet 501 is placed on a copper plate (referred to as “Au plate”) 502plated with gold. The whole thereof is placed on a weigher 506. A copperrod 503 having an outer diameter of 2 mm is used as a probe to apply aload. A resistance value of the anisotropically conductive sheet wasmeasured by a four probe method. Reference numeral 504 indicates aconstant-current power source, and reference numeral 505 denotes avoltmeter.

Example 1

A porous PTFE base having an area of 10 cm², a porosity of 60%, anaverage pore diameter of 0.1 μm and a thickness of 0.5 mm was provided.This porous PTFE base is an expanded PTFE sheet produced by an expandingprocess and having a microstructure comprising fibrils and nodesconnected to each other by the fibrils.

After the expanded PTFE sheet was dipped in ethanol to subject it to ahydrophilization treatment, the thus-treated sheet was impregnated withwater and cooled to 0° C. or lower to solidify water. The expanded PTFEsheet with the solidified water filled into the porous structure thereofwas perforated by means of a combination of a punch and a die, whichforms through-holes having a diameter of 250 μm. A perforating rate was100 holes/min. After the perforating, the temperature of the sheet wasreturned to ordinary temperature, and water was removed by drying.

A perforated portion of the perforated sheet was observed through amicroscope. As a result, it was found that peripheries of theperforations were not collapsed as illustrated in FIG. 1, and the innerwall surfaces of the perforations also had almost a surface cut alongthe punching surface. The peripheries of the perforations were the sameporous as in other portions than the perforated portions, and no changein the microstructure was observed.

Example 2

After the same expanded PTFE sheet as that used in Example 1 wasprovided, impregnated with water and cooled to a temperature not higherthan the solidifying point of water, the sheet was perforated by meansof a blanking blade produced in such a manner that the diameter ofthrough-holes formed is 1 mm. A perforating rate was 100 through-holes/4minutes. After the perforating, the temperature of the sheet wasreturned to ordinary temperature, and water was removed by drying. Aperforated portion of the perforated sheet was then observed. As aresult, neither deformation nor burr was observed at peripheries of theperforations like Example 1, and the microstructure of the peripheriesof the perforations also retained the same form as other portions thanthe perforated portions.

Example 3

After the same expanded PTFE sheet as that used in Example 1 wasprovided, impregnated with water and cooled to a temperature not higherthan the solidifying point of water, the sheet was perforated by meansof a drill controlled in such a manner that the diameter ofthrough-holes formed is 250 μm. At this time, the number of revolutionsof the drill was 100,000 rpm. A perforating rate was 100 through-holes/2minutes. After the perforating, the temperature of the sheet wasreturned to ordinary temperature, and water was removed by drying. Aperforated portion of the perforated sheet was then observed. As aresult, no burr was observed at peripheries of the perforations likeExample 1, and no collapse of the periphery of the perforated portionwas also observed.

Example 4

After the same expanded PTFE sheet as that used in Example 1 wasprovided, impregnated with water and cooled to a temperature not higherthan the solidifying point of water, the sheet was perforated by meansof a blanking blade produced in such a manner that the diameter ofthrough-holes formed is 1 mm. Upon the perforating, ultrasonic vibrationof 40 kHz and 25 W was applied to the tip of the blade.

A perforating rate was 100 through-holes/2 minutes. It took only a halfcompared with Example 2 to operate the sheet. After the perforating, thetemperature of the sheet was returned to ordinary temperature, and waterwas removed by drying. The perforations were then observed. As a result,neither collapse nor burr was observed at peripheries of theperforations, and the edges of the perforations were finished smoothlyand sharply.

Example 5

The same expanded PTFE sheet as that used in Example 1 was provided, andan acetone solution of polymethyl methacrylate (PMMA) was separatelyprovided. The acetone solution was placed in a container having a sizethat the PTFE sheet can be horizontally dipped, and impregnated into theexpanded PTFE sheet by a dipping method. Acetone was removed by dryingto form a composite sheet of PTFE-PMMA. This composite sheet wasperforated at 100,000 rpm by means of a drill that the diameter ofthrough-holes formed is 250 μm. It took 4 minutes to form 100perforations. After completion of the perforating, the expanded PTFEsheet was dipped in acetone to dissolve out PMMA, thereby obtaining aperforated expanded PTFE sheet. The perforations were observed through amicroscope. As a result, there was no change in the microstructure atperipheries of the perforations, and neither collapse nor burr wasobserved at the edges of the perforations.

Comparative Example 1

The same expanded PTFE sheet as that used in Example 1 was provided, andthe sheet was perforated in an intact state by means of a punch and adie, which form through-holes having a diameter of 500 μm. Theperforating time was such that 100 perforations were formed in a minutelike Example 1. After the operation, the perforations were observed. Asa result, burr occurred, and the hole diameter of 500 μm was notretained. In addition, the microstructure of peripheries of theperforations was partially in a twitched state, and so the porousstructure was not retained.

Comparative Example 2

The same expanded PTFE sheet as that used in Example 1 was provided, andthe sheet was perforated in an intact state by means of the sameblanking blade as that used in Example 2. The perforating time was suchthat 100 perforations were formed in 4 minutes like Example 2. After theoperation, the perforations were observed. As a result, burr occurred.In addition, the peripheries of the perforations were in a collapsedstate, and it was observed that the microstructure of that portion wasclearly different from other portions.

Comparative Example 3

The same expanded PTFE sheet as that used in Example 1 was provided, andthe sheet was perforated in an intact state by means of a drill. Thedrill used was the same as that used in Example 3. The number ofrevolutions was also 100,000 rpm likewise. The perforating time was suchthat 100 perforations were formed in 2 minutes like Example 3. After theperforating, the perforations were observed. As a result, it wasobserved that burr occurred, and moreover the resin was twitched due tothe revolution, and the peripheries of the perforations were in acollapsed state.

Example 6

Twenty-five grams of a methacrylic resin (PMMA; product of SumitomoChemical Co., Ltd., trade name “LG6A”) was dissolved in 75 g of acetoneat room temperature to prepare a solution of the methacrylic resin. Anexpanded PTFE sheet (product of SUMITOMO ELECTRIC FINE POLYMER, INC.,trade name “HP-010-30”; bubble point with isopropyl alcohol: 150 kPa;porosity: 60%) cut out into a 2 cm square was slowly dipped in themethacrylic resin solution while taking care that air is not left withinthe porous structure thereof. After it was confirmed that the expandedPTFE sheet became translucent, and the methacrylic resin-solution wascompletely impregnated into the porous structure thereof, it was takenout and air-dried for about 18 hours at room temperature. Through-holes(perforations) were formed at a plurality of positions in the compositesheet thus obtained at 100,000 rpm by means of a drill having a diameterof 250 μm.

After the composite sheet, in which the through-holes had been formed,was then dipped for 1 minute in ethanol to hydrophilize it, the sheetwas dipped for 4 minutes at a temperature of 60° C. in Melplate PC-321produced by Meltex Inc., which was diluted to 100 ml/L, to conductconditioning. After the composite sheet was further dipped for 1 minutein 10% sulfuric acid, it was dipped for 2 minutes in a solution withEnplate PC-236 produced by Meltex Inc. dissolved in a proportion of 180g/L in 0.8% hydrochloric acid as predipping.

The composite sheet was further dipped for 5 minutes in a solution withEnplate PC-236 produced by Meltex Inc. dissolved in a proportion of 150g/L in an aqueous solution of 3% Enplate Activator 444 produced byMeltex Inc., 1% Enplate Activator Additive and 3% hydrochloric acid toapply tin-palladium colloid particles to the surfaces and wall surfacesof the through-holes of the composite sheet.

The composite sheet thus treated was then dipped in acetone to extract(dissolve and remove) the methacrylic resin impregnated into theexpanded PTFE sheet, thereby obtaining an expanded PTFE sheet withpalladium-tin particles applied only to the wall surfaces of thethrough-holes in the expanded PTFE sheet. The sheet thus obtained wasfurther dipped in a liquid obtained by diluting PA-360 produced byMeltex Inc. in a proportion of 50 ml/L with purified water to dissolvetin, thereby activating the catalyst.

The thus-treated expanded PTFE sheet was immersed for 30 minutes in anelectroless copper plating solution prepared with each 5% of MelplateCu-3000A, Melplate Cu-3000B, Melplate Cu-3000C and Melplate Cu-3000D,and 1% of Melplate Cu-3000 Stabilizer, which were all products of MeltexInc., while sufficiently conducting air stirring, thereby depositingcopper particles only on the wall surfaces of the through-holes to makethem conductive. The copper particles were then plated with gold for thepurpose of improving rust prevention and contacting ability withdevices. With respect to the gold plating, a gold displacement platingmethod from nickel was adopted in accordance with the following process.

After the expanded PTFE sheet with the copper particles applied to thewall surfaces of the perforations was dipped for 3 minutes in ActivatorOrotech SIT Additive (80 ml/L) produced by Atotech as predipping, thesheet was dipped for 1 minute in a solution prepared by ActivatorOrotech SIT Activator Concentrate (125 mg/L) produced by Atotech andActivator Orotech SIT Additive (80 ml/L) produced by Atotech forapplying a catalyst, and further dipped for 1 minute in Orotech SITPostdip (25 mg/L) produced by Atotech to apply a palladium catalyst tothe copper particles.

The expanded PTFE sheet was then dipped for 5 minutes in an electrolessnickel plating solution prepared by sodium hyposulfite (20 g/L),trisodium citrate (40 g/L), ammonium borate (13 g/L) and nickel sulfate(22 g/L) to coat the copper particles with nickel.

Thereafter, the expanded PTFE sheet was dipped for 5 minutes in a golddisplacement plating solution [Melplate AU-6630A (200 ml/L), MelplateAU-6630B (100 ml/L), Melplate AU-6630C (20 g/L) and aqueous solution ofsodium gold sulfite (10 g/L in terms of gold)] produced by Meltex Inc.to coat the copper particles with gold, thereby obtaining ananisotropically conductive sheet by an expanded PTFE sheet with only thewall surfaces of the through-holes of 1.00 mm made conductive.

The anisotropically conductive sheet comprising the expanded PTFE sheetobtained in the above-described manner as a base film was cut into 10 mmsquare, and a conduction-starting load of the cut sheet was measured bymeans of the testing equipment shown in FIG. 5. A copper rod having adiameter of 2 mm was used as a probe, and the probe was brought intocontact with an electrode to measure a resistance value by the fourprobe method. As a result, the resistance value was 3.1Ω under apressing load of 5.0 MPa.

Example 7

Paraffin (product of Wako Pure Chemical Industries, Ltd., melting point:68-70° C.) was contained in a stainless steel container placed on a hotplate of 80° C. to melt it. An expanded PTFE sheet (product of SUMITOMOELECTRIC FINE POLYMER, INC., trade name “HP-010-30”; bubble point withisopropyl alcohol: 150 kPa; porosity: 60%) cut out into a 2 cm squarewas slowly dipped in the molten paraffin while taking care that air isnot left within the voids of the porous structure thereof. After it wasconfirmed that the expanded PTFE sheet became translucent, and theparaffin was completely impregnated into the porous structure thereof,it was taken out and air-cooled at room temperature to solidify themolten paraffin. Through-holes (perforations) were formed at a pluralityof positions in the composite sheet thus obtained at 100,000 rpm bymeans of a drill having a diameter of 250 μm.

After the composite sheet, in which the through-holes had been formed,was then dipped for 1 minute in ethanol to hydrophilize it, the sheetwas dipped for 4 minutes at a temperature of 60° C. in Melplate PC-321produced by Meltex Inc., which was diluted to 100 ml/L, to conductconditioning. After the composite sheet was further dipped for 1 minutein 10% sulfuric acid, it was dipped for 2 minutes in a solution withEnplate PC-236 produced by Meltex Inc. dissolved in a proportion of 180g/L in 0.8% hydrochloric acid as predipping.

The composite sheet was further dipped for 5 minutes in a solution withEnplate PC-236 produced by Meltex Inc. dissolved in a proportion of 150g/L in an aqueous solution of 3% Enplate Activator 444 produced byMeltex Inc., 1% Enplate Activator Additive and 3% hydrochloric acid toapply tin-palladium colloid particles to the surfaces and wall surfacesof the through-holes of the composite sheet. The sheet was furtherdipped in a liquid obtained by diluting PA-360 produced by Meltex Inc.in a proportion of 50 ml/L with purified water to dissolve tin, therebyactivating the catalyst.

The composite sheet thus treated was then dipped in acetone to extractand remove the paraffin impregnated into the expanded PTFE sheet,thereby obtaining an expanded PTFE sheet with palladium-tin particlesapplied only to the wall surfaces of the through-holes.

The thus-obtained expanded PTFE sheet was immersed for 30 minutes in anelectroless copper plating solution prepared with each 5% of MelplateCu-3000A, Melplate Cu-3000B, Melplate Cu-3000C and Melplate Cu-3000D,and 1% of Melplate Cu-3000 Stabilizer, which were all products of MeltexInc., while sufficiently conducting air stirring, thereby making onlythe wall surfaces of the through-holes conductive with copper particles.

The copper particles were then coated with gold for the purpose ofimproving rust prevention and contacting ability with devices. Withrespect to the coating by the gold plating, a gold displacement platingmethod from nickel was adopted in accordance with the following process.

After the expanded PTFE sheet with the copper particles applied to thewall surfaces of the perforations was dipped for 3 minutes in ActivatorOrotech SIT Additive (80 ml/L) produced by Atotech as predipping, thesheet was dipped for 1 minute in a solution prepared by ActivatorOrotech SIT Activator Concentrate (125 mg/L) produced by Atotech andActivator Orotech SIT Additive (80 ml/L) produced by Atotech forapplying a catalyst, and further dipped for 1 minute in Orotech SITPostdip (25 mg/L) produced by Atotech to apply a palladium catalyst tothe copper particles.

The expanded PTFE sheet was then dipped for 5 minutes in an electrolessnickel plating solution prepared by sodium hyposulfite (20 g/L),trisodium citrate (40 g/L), ammonium borate (13 g/L) and nickel sulfate(22 g/L) to coat the copper particles with nickel.

Thereafter, the expanded PTFE sheet was dipped for 5 minutes in a golddisplacement plating solution [Melplate AU-6630A (200 ml/L), MelplateAU-6630B (100 ml/L), Melplate AU-6630C (20 g/L) and aqueous solution ofsodium gold sulfite (10 g/L in terms of gold)] produced by Meltex Inc.to coat the copper particles with gold, thereby obtaining ananisotropically conductive sheet by an expanded PTFE sheet with only thewall surfaces of the through-holes of 1.00 mm made conductive.

The anisotropically conductive sheet comprising the expanded PTFE sheetobtained in the above-described manner as a base film was cut into 10 mmsquare, and a conduction-starting load of the cut sheet was measured bymeans of the apparatus shown in FIG. 5. A copper rod having a diameterof 2 mm was used as a probe, and the probe was brought into contact withan electrode to measure a resistance value by the four probe method. Asa result, the resistance value was 3.9Ω under a pressing load of 5.0MPa.

Comparative Example 4

Through-holes (perforations) were formed at a plurality of positions inan expanded PTFE sheet (HP-010-30, product of SUMITOMO ELECTRIC FINEPOLYMER, INC.) at 100,000 rpm by means of a drill having a diameter of250 μm. It was then attempted to make only the wall surfaces of thethrough-holes conductive in the same manner as in Example 1. However,the porous structure (microstructure) of the inner wall surfaces of thethrough-holes was collapsed, so that the copper particles could not beanchored thereto to deposit the copper particles.

Example 8

To 100 g of methyl methacrylate (LIGHT-ESTER M, product of KYOEISHACHEMICAL Co., LTD.) was added 0.2 g of azobisisobutyronitrile (productof Wako Pure Chemical Industries, Ltd.) as a heat polymerizationinitiator, and the resultant mixture was stirred. The same expanded PTFEsheet as that used in Example 1 was provided and impregnated with themethyl methacrylate solution. The expanded PTFE sheet impregnated withthe methyl methacrylate solution was heated at 80° C. for 4 hours on ahot plate to heat-polymerize methyl methacrylate.

After the polymerization, the expanded PTFE sheet was perforated bymeans of a combination of a punch and a die, which forms through-holeshaving a diameter of 250 μm. A perforating rate was 100 holes/min. Afterthe perforating, the polymer (polymethyl methacrylate) of methylmethacrylate was dissolved by means of a Soxhlet extractor using methylethyl ketone as a solvent to remove it by extraction.

The perforations in the perforated sheet obtained in such a manner wereobserved through a microscope. As a result, it was found thatperipheries of the perforations were not collapsed like Example 1, andthe inner wall surfaces of the perforations also had a surface cut alonga punch hole. Neither deformation nor burr was observed at peripheriesof the perforations, and the microstructure of the peripheries of theperforations also retained the same form as other portions than theperforated portions.

Example 9

To 100 g of methyl methacrylate (LIGHT-ESTER M, product of KYOEISHACHEMICAL Co., LTD.) was added 0.2 g of IRGACURE 184 (product of WakoPure Chemical Industries, Ltd.) as a photopolymerization initiator, andthe resultant mixture was stirred. The same expanded PTFE sheet as thatused in Example 1 was provided and impregnated with the methylmethacrylate solution.

The expanded PTFE sheet thus treated was then irradiated withultraviolet rays at 50 mW/cm² for 10 minutes by means of a high pressuremercury lamp to photopolymerize methyl methacrylate. After perforationswere formed in the same manner as in Example 8, the polymer wasextracted and removed with a solvent.

The perforations in the perforated sheet obtained in such a manner wereobserved through a microscope. As a result, it was found thatperipheries of the perforations were not collapsed like Example 1, andthe inner wall surfaces of the perforations also had a surface cut alonga punch hole. Neither deformation nor burr was observed at peripheriesof the perforations, and the microstructure of the peripheries of theperforations also retained the same form as other portions than theperforated portions.

Example 10

Twenty grams of polymethyl methacrylate (SUMIPEX LG35, product ofSumitomo Chemical, Co., Ltd) was dissolved in 80 g of methylmethacrylate (LIGHT-ESTER M, product of KYOEISHA CHEMICAL Co., LTD.)under conditions of 40° C. for 12 hours, 0.2 g of IRGACURE 184 (productof Wako Pure Chemical Industries, Ltd.) was added as aphoto-polymerization initiator, and the resultant mixture was stirred.The same expanded PTFE sheet as that used in Example 1 was provided andimpregnated with the methyl methacrylate solution.

The expanded PTFE sheet thus treated was then irradiated withultraviolet rays at 50 mW/cm² for 10 minutes by means of a high pressuremercury lamp to photopolymerize methyl methacrylate. After perforationswere formed in the same manner as in Example 8, the polymer wasextracted and removed with a solvent.

The perforations in the perforated sheet obtained in such a manner wereobserved through a microscope. As a result, it was found thatperipheries of the perforations were not collapsed like Example 1, andthe inner wall surfaces of the perforations also had a surface cut alonga punch hole. Neither deformation nor burr was observed at peripheriesof the perforations, and the microstructure of the peripheries of theperforations also retained the same form as other portions than theperforated portions.

Example 11

Three expanded PTFE sheets each having an area of 10 cm², a porosity of60%, an average pore diameter of 0.1 μm (bubble point with isopropylalcohol: 150 kPa;) and a thickness of 30 μm were superimposed on oneanother and held between 2 stainless steel plates each having dimensionsof 3 mm in thickness, 150 mm in length and 100 mm in width. A load ofthe stainless steel plates was applied to the sheets, and a heattreatment was conducted at 350° C. for 30 minutes. After the heating,the sheets were quenched with water from above the stainless steelplates to obtain a laminate with the 3 layers of the expanded PTFEsheets fusion-bonded to one another.

To 100 g of methyl methacrylate (LIGHT-ESTER M, product of KYOEISHACHEMICAL Co., LTD.) was added 0.2 g of azobisisobutyronitrile (productof Wako Pure Chemical Industries, Ltd.) as a heat polymerizationinitiator, and the resultant mixture was stirred. The laminate preparedabove was impregnated with the methyl methacrylate solution. Thelaminate impregnated with the methyl methacrylate solution was heated at80° C. for 4 hours on a hot plate to heat-polymerize methylmethacrylate.

After the polymerization, the laminate was perforated by means of acombination of a punch and a die, which forms through-holes having adiameter of 250 μm. A perforating rate was 100 holes/min. After theperforating, the polymer (polymethyl methacrylate) of methylmethacrylate was dissolved by means of a Soxhlet extractor using methylethyl ketone as a solvent to remove it by extraction.

After the laminate was dipped for 1 minute in ethanol to hydrophilizeit, the laminate was dipped for 4 minutes at a temperature of 60° C. inMelplate PC-321 produced by Meltex Inc., which was diluted to 100 ml/L,to conduct conditioning. After the laminate was further dipped for 1minute in 10% sulfuric acid, it was dipped for 2 minutes in a solutionwith Enplate PC-236 produced by Meltex Inc. dissolved in a proportion of180 g/L in 0.8% hydrochloric acid as predipping.

The laminate was dipped for 5 minutes in a solution with Enplate PC-236produced by Meltex Inc. dissolved in a proportion of 150 g/L in anaqueous solution of 3% Enplate Activator 444 produced by Meltex Inc., 1%Enplate Activator Additive and 3% hydrochloric acid to applytin-palladium colloid particles to the surfaces and wall surfaces of thethrough-holes of the laminate. The laminate thus treated was then dippedin a liquid obtained by diluting PA-360 produced by Meltex Inc. in aproportion of 50 ml/L with purified water to dissolve tin, therebyactivating the catalyst. Thereafter, the mask layers on both surfaceswere separated to obtain an expanded PTFE sheet (base film) with thecatalyst palladium particles applied only to the wall surfaces of thethrough-holes.

The thus-obtained base film was immersed for 20 minutes in anelectroless copper plating solution prepared with each 5% of MelplateCu-3000A, Melplate Cu-3000B, Melplate Cu-3000C and Melplate Cu-3000D,and 1% of Melplate Cu-3000 Stabilizer, which were all products of MeltexInc., while sufficiently conducting air stirring, thereby making onlythe wall surfaces of the perforations conductive with copper particles.The base film was further dipped for 30 seconds in Entech Cu-56 producedby Meltex Inc., which was prepared at 5 ml/L to subject the copperparticles to a rust-preventing treatment, thereby obtaining ananisotropically conductive sheet comprising the expanded PTFE sheet as abase film.

In the plating process, water washing with purified water was conductedfor about 30 seconds to 1 minute after the respective dipping stepsother than the dipping between the predipping step and thecatalyst-applying step of the electroless copper plating. The respectivesteps were all conducted at ordinary temperature (20 to 30° C.) exceptfor the conditioning.

The anisotropically conductive sheet comprising the expanded PTFE sheetobtained in the above-described manner as a base film was cut into 10 mmsquare, and a conduction-starting load of the cut sheet was measured bymeans of the apparatus shown in FIG. 5. A copper rod having a diameterof 3 mm was used as a probe, and the probe was brought into contact withan electrode to measure a resistance value by the four probe method. Asa result, the resistance value was 3.5Ω under a pressing load of 5.0MPa.

INDUSTRIAL APPLICABILITY

According to the present invention, there are provided porous resinbases, in which perforations having smooth and sharp edges and a uniformopening diameter have been formed at necessary positions thereof withoutincurring collapse of the porous structure at peripheral portions of theperforations, including the inner wall surfaces thereof and also withoutproducing burr.

The perforated porous resin bases obtained by the production processesof the present invention are useful as, for example, insulating bases ofmaterials for connection between circuits and anisotropically conductivematerials, and further can be used in a wide variety of fields includingmedical devices such as patch repairing materials, and separationmembranes.

In addition, according to the present invention, the soluble polymer orparaffin is used as a masking material for limiting conductive portionsprovided in the porous resin base, whereby the mask layers coming intohighly close contact with the base can be formed, and it can be easilyremoved after the masking. When the soluble polymer or paraffin isimpregnated into the porous structure, and perforations are formed by amachine-working method after the polymer or paraffin is turned into asolid state, the perforations can be formed with high precision withoutcollapsing the porous structure. This process may also be carried out byusing a compound capable of forming a solid substance by a chemicalreaction, such as a polymerizable monomer.

Further, according to the present invention, only the inner wallsurfaces of the perforations may also be made conductive by a processcomprising arranging porous resin layers as mask layers on both surfacesof the porous resin base to prepare a laminate, impregnating thelaminate with the soluble polymer or paraffin or the compound capableforming a solid substance by a chemical reaction, and forming the solidsubstance.

According to the production processes of the present invention, theperforations can be formed without collapsing the porous structure, sothat plating particles composed of a conductive metal can be applied tothe inner wall surfaces of the perforations so as to be entangled in aresin portion forming the porous structure even when the porous resinbase is formed with a fluorocarbon resin material poor in plateadhesion.

The perforated porous resin bases obtained by the production processesaccording to the present invention can be utilized in a wide variety offields of, for example, materials for connection between circuits,anisotropically conductive materials and insulating materials in anelectronic field; medical devices such as patch repairing materials in amedical field; and separation membranes.

The porous resin bases with the inner wall surfaces of the perforationsmade conductive, which are obtained by the production processesaccording to the present invention, can be used in, for example,electrical connection between circuit devices in semiconductor devices;and tests for electrical reliability, which are carried out in circuitboards, semiconductor wafers and semiconductor packages.

1. A process for producing a perforated porous resin base, whichcomprises the following Steps 1 to 4: (1) Step 1 of impregnating theporous structure of a porous resin base with a liquid or solution; (2)Step 2 of forming a solid substance from the liquid or solutionimpregnated; (3) Step 3 of forming a plurality of perforations extendingthrough from the first surface of the porous resin base having the solidsubstance within the porous structure to the second surface in theporous resin base; and (4) Step 4 of melting or dissolving the solidsubstance to remove it from the interior of the porous structure.
 2. Theproduction process according to claim 1, wherein the porous resin baseis a porous resin sheet.
 3. The production process according to claim 2,wherein the porous resin sheet is an expanded polytetrafluoroethylenesheet having a microstructure comprising fibrils and nodes connected toeach other by the fibrils.
 4. The production process according to claim1, wherein in Step 1, the liquid or solution is impregnated into theporous structure of the porous resin base by a casting or dippingmethod.
 5. The production process according to claim 1, wherein theliquid used in Step 1 is a substance having a solidifying point ormelting point within a range of from −150 to 150° C.
 6. The productionprocess according to claim 5, wherein the substance having a solidifyingpoint or melting point within the range of from −150 to 150° C. iswater, an alcohol, a hydrocarbon, a polymer or a mixture of two or morecompounds thereof.
 7. The production process according to claim 5,wherein the substance having a solidifying point or melting point withinthe range of from −150 to 150° C. is a paraffin having a melting pointof at least 15° C.
 8. The production process according to claim 1,wherein the substance having a solidifying point or melting point withinthe range of from −150 to 150° C. is impregnated as a liquid at atemperature exceeding the solidifying point or melting point thereofinto the porous structure of the porous resin base in Step 1, the solidsubstance is solidified at a temperature not higher than the solidifyingpoint or melting point to form a solid substance in Step 2, and thissubstance is melted at a temperature exceeding the solidifying point ormelting point to remove it in Step
 4. 9. The production processaccording to claim 1, wherein the liquid used in Step 1 is a solution ofa soluble polymer or paraffin.
 10. The production process according toclaim 9, wherein the soluble polymer or paraffin is in a solid state ata temperature ranging from 15 to 30° C.
 11. The production processaccording to claim 1, wherein the solution of the soluble polymer orparaffin is impregnated into the porous structure of the porous resinbase in Step 1, the solvent is vaporized out to form a solid substanceof the polymer or paraffin in Step 2, and the solid substance isdissolved with a solvent to remove it in Step
 4. 12. The productionprocess according to claim 1, wherein the liquid or solution used inStep 1 is a liquid or solution containing a compound capable of forminga solid substance by a chemical reaction.
 13. The production processaccording to claim 12, wherein the compound capable of forming a solidsubstance by a chemical reaction is a polymerizable monomer undergoing apolymerization reaction by heat or light to form a polymer.
 14. Theproduction process according to claim 13, wherein the liquid or solutioncontaining the compound capable of forming a solid substance by achemical reaction is a liquid or solution also containing, in additionto the polymerizable monomer, a polymer obtained by the polymerizationof the polymerizable monomer.
 15. The production process according toclaim 13, wherein the polymerizable monomer is an acrylate ormethacrylate.
 16. The production process according to claim 1, whereinthe liquid or solution containing the compound capable of forming asolid substance by a chemical reaction is impregnated into the porousstructure of the porous resin base in Step 1, the compound is subjectedto a chemical reaction to form a solid substance in Step 2, and thesolid substance is dissolved with a solvent to remove it in Step
 4. 17.The production process according to claim 1, wherein in the perforatingStep 3, the perforations are formed by means of i) a mechanicallyperforating method, ii) a method of etching by a light-abrasion method,or iii) a method of perforating by using an ultrasonic head equippedwith at least one oscillator on the tip thereof and pressing the tip ofthe oscillator against the porous resin base to apply ultrasonic energythereto.
 18. A process for producing a porous resin base with the innerwall surfaces of perforations made conductive, which comprises thefollowing Steps I to VI: (1) Step I of impregnating the porous structureof a porous resin base including both surfaces thereof with a solublepolymer or paraffin, or a compound capable of forming a solid substanceby a chemical reaction; (2) Step II of forming a solid substance fromthe soluble polymer or paraffin, or the compound capable of forming asolid substance by a chemical reaction, which has been impregnated, toform a composite sheet of a structure that both surfaces of the porousresin base have a layer of the solid substance, and the solid substanceis impregnated into the porous structure; (3) Step III of forming aplurality of perforations extending through from the first surface ofthe composite sheet to the second surface in the composite sheet; (4)Step IV of applying a catalyst facilitating a reducing reaction of ametal ion to the surfaces of the composite sheet including the innerwall surfaces of the respective perforations; (5) Step V of removing thesolid substance from the composite sheet; and (6) Step VI of using thecatalyst applied to and remaining on the inner wall surfaces of therespective perforations in the porous resin base to apply a conductivemetal to the inner wall surfaces.
 19. The production process accordingto claim 18, wherein the porous resin base is a porous resin sheet. 20.The production process according to claim 19, wherein the porous resinsheet is an expanded polytetrafluoroethylene sheet having amicrostructure comprising fibrils and nodes connected to each other bythe fibrils.
 21. The production process according to claim 18, whereinthe soluble polymer or paraffin is in a solid state at a temperatureranging from 15 to 30° C.
 22. The production process according to claim18, wherein in Step I, the soluble polymer or paraffin is impregnated bya method of casting a solution or melt of the soluble polymer orparaffin on both surfaces of the porous resin base or dipping the porousresin base in a solution or melt of the soluble polymer or paraffin, andin Step II, a composite sheet of the structure that both surfaces of theporous resin base have a solid layer of the soluble polymer or paraffin,and the solid soluble polymer or paraffin is impregnated into the porousstructure is formed by a method of vaporizing out the solvent orlowering the temperature of the porous resin base to a temperature nothigher than the solidifying point or melting point of the solublepolymer or paraffin.
 23. The production process according to claim 18,wherein in Step I, the compound capable of forming a solid substance bya chemical reaction is impregnated by a method of casting a liquid orsolution containing, as the compound capable of forming a solidsubstance by a chemical reaction, a polymerizable monomer undergoing apolymerization reaction by heat or light to form a polymer on bothsurfaces of the porous resin base or dipping the porous resin base inthe liquid or solution containing the polymerizable monomer undergoing apolymerization reaction by heat or light to form a polymer, and in StepII, a composite sheet of the structure that both surfaces of the porousresin base have a solid polymer layer, and the solid polymer isimpregnated into the porous structure is formed by a method ofpolymerizing the polymerizable monomer by heat or light to form a solidpolymer.
 24. The production process according to claim 18, wherein inStep III, the plurality of the perforations are formed in the compositesheet by means of i) a mechanically perforating method, ii) a method ofetching by a light-abrasion method, or iii) a method of perforating byusing an ultrasonic head equipped with at least one oscillator on thetip thereof and pressing the tip of the oscillator against the porousresin base to apply ultrasonic energy thereto.
 25. The productionprocess according to claim 18, wherein in Step V, the solid substance isdissolved and removed by using a solvent that does not dissolve orhardly dissolves the porous resin base, but exhibits good solubility forthe solid substance.
 26. The production process according to claim 18,wherein in Step IV, the solid substance is melted and removed.
 27. Theproduction process according to claim 18, wherein in Step VI, theconductive metal is applied to the inner wall surfaces of the respectiveperforations by electroless plating.
 28. The production processaccording to claim 18, wherein the porous resin base with the inner wallsurfaces of the perforations made conductive is an anisotropicallyconductive sheet that has conductive portions formed by the conductivemetal applied to the resin portion of the porous structure in the innerwall surfaces of the plurality of the perforations extending throughfrom the first surface to the second surface and permits impartingconductivity only to the thickness-wise direction of the sheet by theconductive portions.
 29. A process for producing a porous resin basewith the inner wall surfaces of perforations made conductive, whichcomprises the following Steps i to viii: (1) Step i of laminating, asmask layers, porous resin layers (B) and (C) on both surfaces of aporous resin base (A) to form a laminate of a 3-layer structure; (2)Step ii of impregnating the respective porous structures of the laminatewith a soluble polymer or paraffin, or a compound capable of forming asolid substance by a chemical reaction; (3) Step iii of forming a solidsubstance from the soluble polymer or paraffin, or the compound capableof forming a solid substance by a chemical reaction, which has beenimpregnated; (4) Step iv of forming a plurality of perforationsextending through from the first surface of the laminate having thesolid substance within the respective porous structures to the secondsurface in the laminate; (5) Step v of dissolving the solid substance toremove it from the interiors of the respective porous structures; (6)Step vi of applying a catalyst facilitating a reducing reaction of ametal ion to the surfaces of the laminate including the inner wallsurfaces of the respective perforations; (7) Step vii of removing themask layers from both surfaces of the porous resin base (A); and (8)Step viii of using the catalyst applied to and remaining on the innerwall surfaces of the respective perforations in the porous resin base(A) to apply a conductive metal to the inner wall surfaces.
 30. Theproduction process according to claim 29, wherein the porous resin baseis a porous resin sheet.
 31. The production process according to claim30, wherein the porous resin sheet is an expandedpolytetrafluoroethylene sheet having a microstructure comprising fibrilsand nodes connected to each other by the fibrils.
 32. The productionprocess according to claim 29, wherein the compound capable of forming asolid substance by a chemical reaction is a polymerizable monomerundergoing a polymerization reaction by heat or light to form a polymer.33. The production process according to claim 32, wherein the liquid orsolution containing the compound capable of forming a solid substance bya chemical reaction is a liquid or solution also containing, in additionto the polymerizable monomer, a polymer obtained by the polymerizationof the polymerizable monomer.
 34. The production process according toclaim 32, wherein the polymerizable monomer is an acrylate ormethacrylate.
 35. The production process according to claim 29, whereinin Step ii, the soluble polymer or paraffin is impregnated by casting asolution or melt of the soluble polymer or paraffin on both surfaces ofthe laminate or dipping the laminate in a solution or melt of thesoluble polymer or paraffin, and in Step iii, a solid polymer orparaffin is formed by a method of vaporizing out the solvent or loweringthe temperature of the laminate to a temperature not higher than thesolidifying point or melting point of the soluble polymer or paraffin.36. The production process according to claim 29, wherein in Step ii, aliquid or solution containing, as the compound capable of forming asolid substance by a chemical reaction, a polymerizable monomerundergoing a polymerization reaction by heat or light to form a polymeris impregnated into the respective porous structures of the laminate,and in Step iii, the polymerizable monomer is polymerized by heat orlight to form a solid polymer.
 37. The production process according toclaim 29, wherein in Step iv, the plurality of the perforations areformed in the laminate by means of i) a mechanically perforating method,ii) a method of etching by a light-abrasion method, or iii) a method ofperforating by using an ultrasonic head equipped with at least oneoscillator on the tip thereof and pressing the tip of the oscillatoragainst the porous resin base to apply ultrasonic energy thereto. 38.The production process according to claim 29, wherein in Step v, thesolid substance is dissolved and removed by using a solvent that doesnot dissolve or hardly dissolves the porous resin base, but exhibitsgood solubility for the solid substance.
 39. The production processaccording to claim 29, wherein in Step viii, the conductive metal isapplied to the inner wall surfaces of the respective perforations byelectroless plating.
 40. The production process according to claim 29,wherein the porous resin base with the inner wall surfaces of theperforations made conductive is an anisotropically conductive sheet thathas conductive portions formed by the conductive metal applied to theresin portion of the porous structure in the inner wall surfaces of theplurality of the perforations extending through from the first surfaceto the second surface and permits imparting conductivity only to thethickness-wise direction of the sheet by the conductive portions.